offscreen: Notify on the correct property name
offscreen: Notify on the correct property name

When renaming 'enabled' to 'redirect-enabled', the g_object_notify call
was left the same. Correct this to fix notification and run-time
warnings.

jonnylamb/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx