main: Add override for dynamic-workspaces key

https://bugzilla.gnome.org/show_bug.cgi?id=671568

data/org.gnome.shell.gschema.xml.in

@@ -172,6 +172,14 @@ value here is from the GsmPresenceStatus enumeration.</_summary>
       </description>
     </key>
 
+    <key name="dynamic-workspaces" type="b">
+      <default>true</default>
+      <summary>Workspaces are managed dynamically</summary>
+      <description>
+        This key overrides the key in org.gnome.mutter when running GNOME Shell.
+      </description>
+    </key>
+
     <key name="workspaces-only-on-primary" type="b">
       <default>true</default>
       <summary>Workspaces only on primary monitor</summary>