netfilter: ipt_CLUSTERIP: make symbol 'cip_netdev_notifier' static

Fixes the following sparse warnings:

net/ipv4/netfilter/ipt_CLUSTERIP.c:867:23: warning:
 symbol 'cip_netdev_notifier' was not declared. Should it be static?

Fixes: 5a86d68b ("netfilter: ipt_CLUSTERIP: fix deadlock in netns exit routine")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>