data: Add [Install] section to rygel.service
data: Add [Install] section to rygel.service

This allows running `systemctl --user enable rygel` in order to
automatically launch rygel at login. The default behavior is not
changed.

If lingering is also enabled for the user, rygel will be started
at boot; this will then share media without needing a session.

Since this disregards whether GSD would have launched rygel, it
is primarily useful for headless servers and avoids the wrap-dbus
hack used by the example system service.

https://bugzilla.gnome.org/show_bug.cgi?id=766332

GNOME/rygel,GNOME/rygel,GNOME/rygel