LibGUI: Add a few missing classes to the forwarding header

Author: https://github.com/alimpfard
Commit: 6eeb8225f8
Pull-request: https://github.com/SerenityOS/serenity/pull/5866
Issue: https://github.com/SerenityOS/serenity/issues/4269
Issue: https://github.com/SerenityOS/serenity/issues/4821
Issue: https://github.com/SerenityOS/serenity/issues/5550
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/Forward.h

@@ -39,6 +39,7 @@ class AutocompleteProvider;
 class BoxLayout;
 class Button;
 class CheckBox;
+class ComboBox;
 class Command;
 class DragEvent;
 class DropEvent;
@@ -68,6 +69,7 @@ class MultiView;
 class OpacitySlider;
 class PaintEvent;
 class Painter;
+class RadioButton;
 class ResizeCorner;
 class ResizeEvent;
 class ScrollBar;