bch: don't use __BSD_VISIBLE to test for fls

Commit 4ecc9883 assumes fls is in libc
if __BSD_VISIBLE is defined.  This appears to only be true on FreeBSD
and DragonFlyBSD.  OpenBSD defines __BSD_VISIBLE and does not have fls
in strings.h/libc.

Switch the test for __BSD_VISIBLE to one for __DragonFly__ and
__FreeBSD__ to unbreak the build on OpenBSD.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>