netfilter: nf_nat_proto: make tables static

Sparse warns about two tables not being declared.

  CHECK   net/netfilter/nf_nat_proto.c
net/netfilter/nf_nat_proto.c:725:26: warning: symbol 'nf_nat_ipv4_ops' was not declared. Should it be static?
net/netfilter/nf_nat_proto.c:964:26: warning: symbol 'nf_nat_ipv6_ops' was not declared. Should it be static?

And in fact they can indeed be static.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>