add __isoc99_vfscanf weak alias to vfscanf

this glibc abi compatibility function was missed when the scanf
aliases were added.

src/stdio/vfscanf.c

@@ -328,3 +328,5 @@ match_fail:
 	FUNLOCK(f);
 	return matches;
 }
+
+weak_alias(vfscanf,__isoc99_vfscanf);