$ 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.