LibGUI: Forward declare ImageWidget

Author: https://github.com/trflynn89
Commit: https://github.com/SerenityOS/serenity/commit/4cf4a7cc87f
Pull-request: https://github.com/SerenityOS/serenity/pull/12263
Reviewed-by: https://github.com/davidot ✅

Userland/Libraries/LibGUI/Forward.h

@@ -34,6 +34,7 @@ class HorizontalBoxLayout;
 class HorizontalSlider;
 class Icon;
 class IconView;
+class ImageWidget;
 class JsonArrayModel;
 class KeyEvent;
 class Label;