net-firewall/shorewall: Runtime dependency of net-firewall/iptables bumped

net-firewall/shorewall*-4.5.19 introduced a new 'event' and 'trigger'
functionality using ip[6]tables 'recent' module, which uncovered an
iptables bug:

  http://thread.gmane.org/gmane.comp.security.shorewall/29850/focus=29859

This bugfix was included in iptables-1.4.20. Therefore we bump
net-firewall/shorewall* runtime dependency of net-firewall/iptables for
>=net-firewall/shorewall*-4.5.19 to version 1.4.20.

See bug #478224.