gtp: change NET_UDP_TUNNEL dependency to select

Similarly to commit a7603ac1 ("geneve: change NET_UDP_TUNNEL
dependency to select"), GTP has a dependency on NET_UDP_TUNNEL which
makes impossible to compile it if no other protocol depending on
NET_UDP_TUNNEL is selected.

Fix this by changing the depends to a select, and drop NET_IP_TUNNEL from
the select list, as it already depends on NET_UDP_TUNNEL.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>