net-firewall/shorewall: SRC_URI changed to primary mirror

With the release of shorewall-4.5.20 it was shown, that shorewall's
mirror system can be broken:

http://thread.gmane.org/gmane.comp.security.shorewall/29950/focus=29956

Using the primary mirror in the ebuild shouldn't be a problem, because
Gentoo will mirror the files from shorewall's primary mirror too, so we
are not causing load on shorewall's primary mirror.
net-firewall/shorewall: RepoMan satisfied.