Instead of trying to handle ffs() with the normal rename-mark-hidden-and-alias

dance, mark it protected.  This works better for both gcc and clang: gcc
blocks overriding of internal calls, while clang permits inlining again.
ok otto@

src/lib/libc/string/ffs.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ffs.c,v 1.9 2015/08/31 02:53:57 guenther Exp $	*/
+/*	$OpenBSD: ffs.c,v 1.10 2018/01/18 08:23:44 guenther Exp $	*/
 
 /*
  * Public domain.
@@ -38,4 +38,3 @@ ffs(int mask)
 
 	return (bit + t[ r & 0xf ]);
 }
-DEF_WEAK(ffs);