pam_lastlog2: fix libpam linking in autotools build

pam_lastlog2: fix libpam linking in autotools build

Move -lpam from LDFLAGS to LIBADD. When -lpam is in LDFLAGS it
appears on the linker command line before object files, so the
--as-needed linker flag (default on Fedora) discards it before
seeing any undefined PAM symbols. This results in pam_lastlog2.so
missing libpam.so in its ELF NEEDED entries.

The module then fails to load with dlopen() if the calling process
does not itself link against libpam (e.g., systemd in Fedora 44+):

  PAM unable to dlopen(pam_lastlog2.so): undefined symbol: pam_syslog

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=2453457
Signed-off-by: Karel Zak <kzak@redhat.com>