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>