olsrd: add iptables as dependency

olsrd: add iptables as dependency

The upcoming OpenWrt version (currently in the master branch) ships
firewall4 by default [1], which uses nftables instead of iptables.

Users might choose to use a compatible layer [2] with iptables-nft,
decide which iptables they want to use, and make appropriate changes
related to fw3/f4[3].

This package requires to use iptables (no matter which variant) for
smart-gw-rules.

[1] https://github.com/openwrt/openwrt/commit/08d9f6e3020b4a149b2007b6ed7d684c49af9bbf
[2] https://www.redhat.com/en/blog/using-iptables-nft-hybrid-linux-firewall
[3] https://github.com/openwrt/openwrt/commit/795e7155cbe3e78669f6821bf7aecb7c4e1e1afb

Signed-off-by: Nick Hainke <vincent@systemli.org>