kerberos5: Silence compiler warnings

Building the kerberos5 subdirectory currently produces lots of warnings.
Since there are many instances of these warnings and it's contrib code,
this change silences the warnings instead of fixing them.

Reviewed By: jhb, cy, bjk
Differential Revision: https://reviews.freebsd.org/D28025

(cherry picked from commit 4f009328a2477f5f1c682629fc5708498b7c689f)