monitor-manager/xrandr: Chain up constructed vfunc

'monitor-manager: Keep pointer to backend' added a
GObjectClass::constructed implementation, but didn't chain it up to the
parent. Fix that.

https://bugzilla.gnome.org/show_bug.cgi?id=788921

src/backends/x11/meta-monitor-manager-xrandr.c

@@ -919,6 +919,8 @@ meta_monitor_manager_xrandr_constructed (GObject *object)
       meta_monitor_manager_xrandr_init_monitors (manager_xrandr);
 #endif
     }
+
+  G_OBJECT_CLASS (meta_monitor_manager_xrandr_parent_class)->constructed (object);
 }
 
 static void