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>