gnome-shell.modules: add libcanberra as a dependency of Mutter

libcanberra is an (optional) dependency of Mutter; since we have
it in the moduleset we need to mark the dependency so things get
built in the right order.

tools/build/gnome-shell.modules

@@ -150,6 +150,7 @@
       <dep package="clutter"/>
       <dep package="gconf"/>
       <dep package="gtk3"/>
+      <dep package="libcanberra"/>
     </dependencies>
   </autotools>