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>

drivers/net/Kconfig

@@ -216,8 +216,8 @@ config GENEVE
 
 config GTP
 	tristate "GPRS Tunneling Protocol datapath (GTP-U)"
-	depends on INET && NET_UDP_TUNNEL
-	select NET_IP_TUNNEL
+	depends on INET
+	select NET_UDP_TUNNEL
 	---help---
 	  This allows one to create gtp virtual interfaces that provide
 	  the GPRS Tunneling Protocol datapath (GTP-U). This tunneling protocol