[XFRM]: Fix IPv4 tunnel mode decapsulation with IPV6=n

Add missing break when CONFIG_IPV6=n.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/xfrm4_mode_tunnel.c

@@ -95,6 +95,7 @@ static int xfrm4_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 
 	switch(iph->protocol){
 		case IPPROTO_IPIP:
+			break;
 #if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
 		case IPPROTO_IPV6:
 			break;