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>