Fix install of iptables pkg-config files.

libiptc.pc depends on libip[4|6]tc.pc, thus all of those need to be
installed.
Should fix collectd build and thus #13146; which should make collectd
appear in snapshots again.

Signed-off-by: Danny Baumann <dannybaumann@web.de>

git-svn-id: svn://svn.openwrt.org/openwrt@36509 3c298f89-4303-0410-b956-a3cf2f4a3e73