strongswan: add missing config files for plugins

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r40524

git-svn-id: svn://svn.openwrt.org/openwrt/branches/packages_12.09@40525 3c298f89-4303-0410-b956-a3cf2f4a3e73