time.h: Fix visibility check for C11 and C23 features.

__BSD_VISIBLE is always defined; we need to check if it's true.

Fixes: d02489d11aed 9b5d724cad10
MFC after: 3 days
Reviewed by: brooks, imp
Differential Revision: https://reviews.freebsd.org/D41733

(cherry picked from commit 8091b82e291642f3034dc3fdc8689a4f4ba027da)