Sorry, I guess I missed the last comment. Seems to still be failing with 5.2.18-dfsg-1 from cosmic-release though.

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-cosmic-canonical-kernel-team-unstable/cosmic/amd64/v/virtualbox/20180824_183905_b1337@/log.gz
https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-cosmic-canonical-kernel-team-unstable/cosmic/amd64/v/virtualbox/20180824_183905_b1337@/artifacts.tar.gz

/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c: In function ‘sf_init_inode’:
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:165:28: error: passing argument 1 of ‘sf_ftime_from_timespec’ from incompatible pointer type [-Werror=incompatible-pointer-types]
     sf_ftime_from_timespec(&inode->i_atime, &info->AccessTime);
                            ^~~~~~~~~~~~~~~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:53:53: note: expected ‘struct timespec *’ but argument is of type ‘struct timespec64 *’
 static void sf_ftime_from_timespec(struct timespec *tv, RTTIMESPEC *ts)
                                    ~~~~~~~~~~~~~~~~~^~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:166:28: error: passing argument 1 of ‘sf_ftime_from_timespec’ from incompatible pointer type [-Werror=incompatible-pointer-types]
     sf_ftime_from_timespec(&inode->i_ctime, &info->ChangeTime);
                            ^~~~~~~~~~~~~~~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:53:53: note: expected ‘struct timespec *’ but argument is of type ‘struct timespec64 *’
 static void sf_ftime_from_timespec(struct timespec *tv, RTTIMESPEC *ts)
                                    ~~~~~~~~~~~~~~~~~^~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:167:28: error: passing argument 1 of ‘sf_ftime_from_timespec’ from incompatible pointer type [-Werror=incompatible-pointer-types]
     sf_ftime_from_timespec(&inode->i_mtime, &info->ModificationTime);
                            ^~~~~~~~~~~~~~~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:53:53: note: expected ‘struct timespec *’ but argument is of type ‘struct timespec64 *’
 static void sf_ftime_from_timespec(struct timespec *tv, RTTIMESPEC *ts)
                                    ~~~~~~~~~~~~~~~~~^~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c: In function ‘sf_setattr’:
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:377:54: error: passing argument 2 of ‘sf_timespec_from_ftime’ from incompatible pointer type [-Werror=incompatible-pointer-types]
             sf_timespec_from_ftime(&info.AccessTime, &iattr->ia_atime);
                                                      ^~~~~~~~~~~~~~~~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:63:69: note: expected ‘struct timespec *’ but argument is of type ‘struct timespec64 *’
 static void sf_timespec_from_ftime(RTTIMESPEC *ts, struct timespec *tv)
                                                    ~~~~~~~~~~~~~~~~~^~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:379:60: error: passing argument 2 of ‘sf_timespec_from_ftime’ from incompatible pointer type [-Werror=incompatible-pointer-types]
             sf_timespec_from_ftime(&info.ModificationTime, &iattr->ia_mtime);
                                                            ^~~~~~~~~~~~~~~~
/var/lib/dkms/virtualbox-guest/5.2.18/build/vboxsf/utils.c:63:69: note: expected ‘struct timespec *’ but argument is of type ‘struct timespec64 *’
 static void sf_timespec_from_ftime(RTTIMESPEC *ts, struct timespec *tv)
                                                    ~~~~~~~~~~~~~~~~~^~