fix gcc warnings

Max Kellermann <max@duempel.org>
diff --git a/extensions/libxt_string.c b/extensions/libxt_string.c
index 50b7f61..f1030bb 100644
--- a/extensions/libxt_string.c
+++ b/extensions/libxt_string.c
@@ -49,7 +49,7 @@
 	{ "algo", 1, NULL, '3' },
 	{ "string", 1, NULL, '4' },
 	{ "hex-string", 1, NULL, '5' },
-	{ }
+	{ .name = NULL }
 };
 
 static void string_init(struct xt_entry_match *m)