If CONFIG_INET_XFRM_TUNNEL is set but CONFIG_IPV6 is n,

net/ipv4/ip_vti.c:493:27: warning: 'vti_ipip6_handler' defined but not used [-Wunused-variable]
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>