$ dpkg-shlibdeps -v debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so
>> Scanning debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so (for Depends field)
Library libpthread.so.0 found in /lib/x86_64-linux-gnu/libpthread.so.0
Library libbsd.so.0 found in /lib/x86_64-linux-gnu/libbsd.so.0
Library libtalloc.so.2 found in /usr/lib/x86_64-linux-gnu/libtalloc.so.2
Library libpam.so.0 found in /lib/x86_64-linux-gnu/libpam.so.0
Library libc.so.6 found in /lib/x86_64-linux-gnu/libc.so.6
Using symbols file /var/lib/dpkg/info/libpam0g:amd64.symbols for libpam.so.0
Using symbols file /var/lib/dpkg/info/libc6:amd64.symbols for libpthread.so.0
Using symbols file /var/lib/dpkg/info/libtalloc2:amd64.symbols for libtalloc.so.2
Using symbols file /var/lib/dpkg/info/libbsd0:amd64.symbols for libbsd.so.0
Using symbols file /var/lib/dpkg/info/libc6:amd64.symbols for libc.so.6
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxLookupName: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxChangeUserPasswordEx: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxCreate: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxInterfaceDetails: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxFree: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxLogonUser: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcFreeMemory: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcAddNamedBlob: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxLookupSid: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcSidToStringBuf: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxLogoffUserEx: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcErrorString: it's probably a plugin
dpkg-shlibdeps: warning: debian/libpam-winbind/lib/x86_64-linux-gnu/security/pam_winbind.so contains an unresolvable reference to symbol wbcCtxGetpwnam: it's probably a plugin
These missing symbols come from libwbclient. Note how wbcCtxFree is among them: that's the missing one you get when you copy the module to /lib/security.
$ dpkg-shlibdeps -v debian/ libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so (for Depends field) 64-linux- gnu/libpthread. so.0 64-linux- gnu/libbsd. so.0 x86_64- linux-gnu/ libtalloc. so.2 64-linux- gnu/libpam. so.0 64-linux- gnu/libc. so.6 dpkg/info/ libpam0g: amd64.symbols for libpam.so.0 dpkg/info/ libc6:amd64. symbols for libpthread.so.0 dpkg/info/ libtalloc2: amd64.symbols for libtalloc.so.2 dpkg/info/ libbsd0: amd64.symbols for libbsd.so.0 dpkg/info/ libc6:amd64. symbols for libc.so.6 libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxLookupName: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxChangeUse rPasswordEx: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxCreate: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxInterface Details: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxFree: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxLogonUser: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcFreeMemory: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcAddNamedBlob: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxLookupSid: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcSidToStringBuf: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxLogoffUserEx: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcErrorString: it's probably a plugin libpam- winbind/ lib/x86_ 64-linux- gnu/security/ pam_winbind. so contains an unresolvable reference to symbol wbcCtxGetpwnam: it's probably a plugin
>> Scanning debian/
Library libpthread.so.0 found in /lib/x86_
Library libbsd.so.0 found in /lib/x86_
Library libtalloc.so.2 found in /usr/lib/
Library libpam.so.0 found in /lib/x86_
Library libc.so.6 found in /lib/x86_
Using symbols file /var/lib/
Using symbols file /var/lib/
Using symbols file /var/lib/
Using symbols file /var/lib/
Using symbols file /var/lib/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
dpkg-shlibdeps: warning: debian/
These missing symbols come from libwbclient. Note how wbcCtxFree is among them: that's the missing one you get when you copy the module to /lib/security.