libauditd: make it a PRIVATELIB

According to the upstream man page (which we don't install), none of
libauditd's symbols are intended to be public. Also, I can't find any
evidence for a port that uses libauditd. Therefore, we should treat it like
other such libraries and use PRIVATELIB.

Reported by: phk
Reviewed by: cem, emaste
MFC after: 2 weeks