LibGUI: Add a few missing classes to the forwarding header

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;