workspace-indicator: Fix window previews

St.Bin no longer expands its child automatically. Unless do we do
explicitly, the preview actor will be allocated with a size of 0x0.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/304>

extensions/workspace-indicator/extension.js

@@ -123,6 +123,8 @@ class WorkspaceThumbnail extends St.Button {
             child: new Clutter.Actor({
                 layout_manager: new WorkspaceLayout(),
                 clip_to_allocation: true,
+                x_expand: true,
+                y_expand: true,
             }),
         });