ip_tunnel: fix sparse non static symbol warning

Fixes the following sparse warning:

net/ipv4/ip_tunnel.c:116:18: warning:
 symbol 'tunnel_dst_check' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -113,7 +113,7 @@ static inline struct dst_entry *tunnel_dst_get(struct ip_tunnel *t)
 	return dst;
 }
 
-struct dst_entry *tunnel_dst_check(struct ip_tunnel *t, u32 cookie)
+static struct dst_entry *tunnel_dst_check(struct ip_tunnel *t, u32 cookie)
 {
 	struct dst_entry *dst = tunnel_dst_get(t);