Make metacity.schemas compliant again

The clutter_plugins schema key did not define the type of the
elements inside the list, thus making gconftool choke when installing
the Metacity schemas.

Since clutter_plugins is a list of strings, the trivial fix is
to add the corresponding <list_type> element to the schema XML.

src/metacity.schemas.in

@@ -361,6 +361,7 @@
       <applyto>/apps/metacity/general/clutter_plugins</applyto>
       <owner>metacity</owner>
       <type>list</type>
+      <list_type>string</list_type>
       <default>[default]</default>
       <locale name="C">
         <short>Clutter Plugins</short>