iptables: enable nftable support by default

iptables: enable nftable support by default

OpenWrt plans to move over to firewall4 which uses nftables under the
hood. To allow a smooth migration the package `iptables-nft` offer a
transparent wrapper to apply iptables rules to nftables.

Without the config option for nftables the package isn't installed and
therefore can't be tested. This commit enabled it and therefore provides
the wrapper.

The size of the iptables package increases from 25436 to 26500 Bytes.

Signed-off-by: Paul Spooren <mail@aparcar.org>