udp: Need to make ip6_udp_tunnel.c have GPL license

Unable to load various tunneling modules without this:

[   80.679049] fou: Unknown symbol udp_sock_create6 (err 0)
[   91.439939] ip6_udp_tunnel: Unknown symbol ip6_local_out (err 0)
[   91.439954] ip6_udp_tunnel: Unknown symbol __put_net (err 0)
[   91.457792] vxlan: Unknown symbol udp_sock_create6 (err 0)
[   91.457831] vxlan: Unknown symbol udp_tunnel6_xmit_skb (err 0)
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_udp_tunnel.c

@@ -103,3 +103,5 @@ int udp_tunnel6_xmit_skb(struct socket *sock, struct dst_entry *dst,
 	return 0;
 }
 EXPORT_SYMBOL_GPL(udp_tunnel6_xmit_skb);
+
+MODULE_LICENSE("GPL");