shadowsocks-libev: ss-rules: request presence of nat table

shadowsocks-libev: ss-rules: request presence of nat table

ss-rules with iptables needs presence of netfilter nat table to work.
ss-rules works before without explicitly requesting it as a dependency
because it's present by default on a pre-firewall4/nftables OpenWrt
install.  We request it explicitly now to make life easier in case
people would like to try ss-rules/iptables on firewall4/nftables enabled
OpenWrt system

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>