auto-move-windows: Fix fallout from API change

MetaWindowActor.get_workspace() was removed, however the now used
MetaWindow.get_workspace() returns a MetaWorkspace rather than an
index, so can't be used directly.

https://bugzilla.gnome.org/show_bug.cgi?id=728820

extensions/auto-move-windows/extension.js

@@ -109,7 +109,7 @@ function myCheckWorkspaces() {
 	if (win.is_on_all_workspaces())
             continue;
 
-	let workspaceIndex = win.get_workspace();
+	let workspaceIndex = win.get_workspace().index();
 	emptyWorkspaces[workspaceIndex] = false;
     }