sit: make function ipip6_valid_ip_proto() static

Fixes the following sparse warning:

net/ipv6/sit.c:1129:6: warning:
 symbol 'ipip6_valid_ip_proto' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -1126,7 +1126,7 @@ static int ipip6_tunnel_update_6rd(struct ip_tunnel *t,
 }
 #endif
 
-bool ipip6_valid_ip_proto(u8 ipproto)
+static bool ipip6_valid_ip_proto(u8 ipproto)
 {
 	return ipproto == IPPROTO_IPV6 ||
 		ipproto == IPPROTO_IPIP ||