Use #if instead of #ifdef for __BSD_VISIBLE tests.

__BSD_VISIBLE is always defined and it's value instead needs to be
tested via #if to determine if FreeBSD-specific APIs should be
exposed.

PR: 196226, 223481 (exp-run)
Submitted by: pluknet
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D12977