net/sched: Export tc_tunnel_key so its UAPI accessible

Export tc_tunnel_key so it can be used from user space.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>