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>

net/netfilter/core.c

@@ -377,7 +377,7 @@ static void nf_remove_net_hook(struct nf_hook_entries *old,
 	}
 }
 
-void __nf_unregister_net_hook(struct net *net, int pf,
+static void __nf_unregister_net_hook(struct net *net, int pf,
 				     const struct nf_hook_ops *reg)
 {
 	struct nf_hook_entries __rcu **pp;