strongswan: move iptables deps to updown plugin

strongswan: move iptables deps to updown plugin

Let's move the iptables IPsec dependencies out of the strongswan package
and into the plugin package that actually depends on it,
strongswan-mod-updown. As the default updown script calls the iptables
binary, also add a dependency on the iptables-legacy package.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>