netfilter: NFT_CHAIN_NAT_IPV* is independent of NFT_NAT

Now that we have masquerading support in nf_tables, the NAT chain can
be use with it, not only for SNAT/DNAT. So make this chain type
independent of it.

While at it, move it inside the scope of 'if NF_NAT_IPV*' to simplify
dependencies.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>