Make the bitcount*() functions unconditionally available, by moving
them out of the #if _BSD_VISIBLE block. Other headers may depend on
bitcount(). The dependencies can be a header not specified by
POSIX, and then namespace restrictions by _XOPEN_SOURCE are not
applicable, as it was reported. Or, we might grow an implementation
of some POSIX facility using __bitcount(), which also should work.

Reported by: Jason Schulz <schulz.j@gmail.com>
Discussed with: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks