xtables-addons: enable necessary kernel options

Enable the required conntrack/netfilter options, otherwise
xtables-addons will fail to build.
The basic iptables options are already covered by the iptables package
which is a required dependency anyway.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>