ip6_vti: Fix build when NET_IP_TUNNEL is not set.

Since commit 469bdcef ip6_vti uses ip_tunnel_get_stats64(),
so we need to select NET_IP_TUNNEL to have this function available.

Fixes: 469bdcef ("ipv6: fix the use of pcpu_tstats in ip6_vti.c")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/Kconfig

@@ -138,6 +138,7 @@ config INET6_XFRM_MODE_ROUTEOPTIMIZATION
 config IPV6_VTI
 tristate "Virtual (secure) IPv6: tunneling"
 	select IPV6_TUNNEL
+	select NET_IP_TUNNEL
 	depends on INET6_XFRM_MODE_TUNNEL
 	---help---
 	Tunneling means encapsulating data of one protocol type within