fix(sys-apps/systemd): Stabilize linux headers 3.13 for systemd

The currently systemd git only compiles against 3.10 headers or later.
Looks like upstream Gentoo is going to stabilize 3.13 so go ahead and
do jump ahead on that to make testing systemd easier.
Merge pull request #583 from marineam/headers

fix(sys-apps/systemd): Stabilize linux headers 3.13 for systemd