st/scroll-view: Add missing notify call

Another property where changes aren't notified when going through
the public setter.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2168>

src/st/st-scroll-view.c

@@ -1228,6 +1228,8 @@ st_scroll_view_set_mouse_scrolling (StScrollView *scroll,
       /* make sure we can receive mouse wheel events */
       if (enabled)
         clutter_actor_set_reactive ((ClutterActor *) scroll, TRUE);
+
+      g_object_notify_by_pspec (G_OBJECT (scroll), props[PROP_MOUSE_SCROLL]);
     }
 }