Fix OpenBSM with GCC with -Wredundant-decls

Upstream change ed47534 consciously added some redundant functional
declarations, and I'm not sure why. AFAICT they were never required. On
FreeBSD, they break the build with GCC (but not Clang) for any program
including libbsm.h with WARNS=6.

Fix by cherry-picking upstream change
https://github.com/openbsm/openbsm/commit/0553c27

Reported by: emaste
Reviewed by: cem
Obtained from: OpenBSM
MFC after: 2 weeks
Pull Request: https://github.com/openbsm/openbsm/pull/31