fix gcc warnings

Max Kellermann <max@duempel.org>
diff --git a/extensions/libip6t_ipv6header.c b/extensions/libip6t_ipv6header.c
index 0d7ffd8..c20fb94 100644
--- a/extensions/libip6t_ipv6header.c
+++ b/extensions/libip6t_ipv6header.c
@@ -150,7 +150,7 @@
 static const struct option ipv6header_opts[] = {
 	{ "header", 1, NULL, '1' },
 	{ "soft", 0, NULL, '2' },
-	{ }
+	{ .name = NULL }
 };
 
 static void ipv6header_init(struct xt_entry_match *m)