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: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>