miro: Fix desktop schema GSettings path.

The real path of the schemas is:

$out/share/gsettings-schemas/gsettings-desktop-schemas-3.10.1/glib-2.0/schemas

While the previous approach was to load schemas from:

$out/share/glib-2.0/schemas

So, we're now relying on the setup hook of glib to find the right schema
path.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>