plugins: fix linkage of GResources

The libcommon_dep gresources were not ending up in the final gsd-* plugin binaries. What this means is that the work to disable the GTK theme parsing at startup was innefective, wasting significant time and memory during application startup.

Collectively, these are still slowing down login.

Calling g_resources_register() at startup is enough to ensure that the the empty gtk.css ends up in the target binary.

You can verify the resources show up in the resulting binary with:

  $ gresources list /path/to/gsd-media-keys
  /org/gtk/libgtk/theme/Disabled/gtk.css

This was found with Sysprof while profiling login on GNOME OS.