ip_vti: Fix unused variable warning

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>

net/ipv4/ip_vti.c

@@ -490,6 +490,7 @@ static struct xfrm_tunnel vti_ipip_handler __read_mostly = {
 	.priority	=	0,
 };
 
+#if IS_ENABLED(CONFIG_IPV6)
 static struct xfrm_tunnel vti_ipip6_handler __read_mostly = {
 	.handler	=	vti_rcv_tunnel,
 	.cb_handler	=	vti_rcv_cb,
@@ -497,6 +498,7 @@ static struct xfrm_tunnel vti_ipip6_handler __read_mostly = {
 	.priority	=	0,
 };
 #endif
+#endif
 
 static int __net_init vti_init_net(struct net *net)
 {