netfilter: nat: make symbol nat_hook static

Fixes the following sparse warning:

net/netfilter/nf_nat_core.c:1039:20: warning:
 symbol 'nat_hook' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_core.c

@@ -1036,7 +1036,7 @@ static struct pernet_operations nat_net_ops = {
 	.size = sizeof(struct nat_net),
 };
 
-struct nf_nat_hook nat_hook = {
+static struct nf_nat_hook nat_hook = {
 	.parse_nat_setup	= nfnetlink_parse_nat_setup,
 #ifdef CONFIG_XFRM
 	.decode_session		= __nf_nat_decode_session,