gre_offload: fix sparse non static symbol warning

Fixes the following sparse warning:

net/ipv4/gre_offload.c:253:5: warning:
 symbol 'gre_gro_complete' 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/gre_offload.c

@@ -250,7 +250,7 @@ static struct sk_buff **gre_gro_receive(struct sk_buff **head,
 	return pp;
 }
 
-int gre_gro_complete(struct sk_buff *skb, int nhoff)
+static int gre_gro_complete(struct sk_buff *skb, int nhoff)
 {
 	struct gre_base_hdr *greh = (struct gre_base_hdr *)(skb->data + nhoff);
 	struct packet_offload *ptype;