libpam: enable multilib

libpam: enable multilib

Fix the following warning:
WARNING: QA Issue: lib32-libpam: Files/directories were installed but not shipped
  /usr/sbin/unix_chkpwd
  /usr/sbin/pam_tally2
  /usr/sbin/mkhomedir_helper
  /usr/sbin/unix_update
  /usr/sbin/pam_timestamp_check
  /usr/sbin/pam_tally

(From OE-Core rev: 1dbb0bc048ceaf4cd2879362ae4a0f1398809349)

Signed-off-by: Constantin Musca <constantinx.musca@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>