system-upgrade-fedora: add rpm selinux plugin

F21 rpm moved the code for handling SELinux labels into a plugin, and
dracut can't automatically pick up plugins, so upgrade.img didn't have
rpm-plugins/selinux.so, so nothing got labels applied and you end up
with mislabeled binaries and such.

So: make sure we have the selinux plugin so RPM does labeling again.

Resolves: rhbz#1146580