static_libpam: Don't install pam.d.5 twice

static_libpam's Makefile includes libpam's Makefile after setting some
variables (like MAN) to empty to avoid installing the manpages twice.
After commit 031e711647c3, it neglected to do this for MANNODEVLINKS,
causing pam.d.5.gz to be installed twice. This is harmless for
installworld, but breaks some things that rely on METALOG (NO_ROOT
installs) since it causes two METALOG entries to be generated for
the same file.

Approved by: re (cperciva)
Fixes: 031e711647c3 ("packages: Install development manpages in the -dev package")
MFC after: 3 days
PR: 290708
Reported by: emaste
Reviewed by: emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53512

(cherry picked from commit a1806e6ff37a606277ab9657b951e918164e9f63)
(cherry picked from commit 92a081bc2d1ad2c06749d5fbcf31697db05a2001)