netfilter: ship nft_chain_nat on 5.1+ kernels

netfilter: ship nft_chain_nat on 5.1+ kernels

The former nft_chain_nat_ipv4 and nft_chain_nat_ipv6 modules have been merged
into a common nft_chain_nat module starting with Linux 5.1.

Ensure that this common module is shipped along with kmod-nft-nat on recent
kernels.

While we're at it, also apply version constraints to other nft modules that
have been merged into the core with newer kernels.

Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2815#comment8016
Signed-off-by: Jo-Philipp Wich <jo@mein.io>