makefs zfs: Disable -Wunused-function for GCC as well.

Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D37469

usr.sbin/makefs/zfs/vdev.c

@@ -38,11 +38,11 @@
 
 #include "zfs.h"
 
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunused-function"
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-function"
 #include "zfs/fletcher.c"
 #include "zfs/sha256.c"
-#pragma clang diagnostic pop
+#pragma GCC diagnostic pop
 
 static void
 blkptr_set(blkptr_t *bp, off_t off, off_t size, uint8_t dntype, uint8_t level,