bch: don't use __BSD_VISIBLE to test for fls

Commit 4ecc988301 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>

lib/bch.c

@@ -117,7 +117,7 @@ struct gf_poly_deg1 {
 };
 
 #ifdef USE_HOSTCC
-#ifndef __BSD_VISIBLE
+#if !defined(__DragonFly__) && !defined(__FreeBSD__)
 static int fls(int x)
 {
 	int r = 32;