fix gcc warnings

Max Kellermann <max@duempel.org>
diff --git a/extensions/libxt_mac.c b/extensions/libxt_mac.c
index 5d84aee..2284d2a 100644
--- a/extensions/libxt_mac.c
+++ b/extensions/libxt_mac.c
@@ -24,7 +24,7 @@
 
 static const struct option mac_opts[] = {
 	{ "mac-source", 1, NULL, '1' },
-	{ }
+	{ .name = NULL }
 };
 
 static void