netfilter: core: make local function __nf_unregister_net_hook static

Fixes the following sparse warning:

net/netfilter/core.c:380:6: warning:
 symbol '__nf_unregister_net_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>