geneve: change NET_UDP_TUNNEL dependency to select

Due to the depends on NET_UDP_TUNNEL, at the moment it is impossible to
compile GENEVE if no other protocol depending on NET_UDP_TUNNEL is
selected.

Fix this 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>
Reviewed-and-tested-by: Andrea Claudi <aclaudi@redhat.com>
Tested-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>