fix gcc warnings

Max Kellermann <max@duempel.org>
diff --git a/extensions/libxt_length.c b/extensions/libxt_length.c
index c55b12b..b812b67 100644
--- a/extensions/libxt_length.c
+++ b/extensions/libxt_length.c
@@ -21,7 +21,7 @@
   
 static const struct option length_opts[] = {
 	{ "length", 1, NULL, '1' },
-	{ }
+	{ .name = NULL }
 };
 
 static u_int16_t