netfilter: nft_hash: fix non static symbol warning

Fixes the following sparse warning:

net/netfilter/nft_hash.c:40:25: warning:
 symbol 'nft_hash_policy' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>