LibGUI: Add GUI::Dialog to the LibGUI forwarding header

Author: https://github.com/trflynn89
Commit: db4fa36b58
Pull-request: https://github.com/SerenityOS/serenity/pull/16091
Reviewed-by: https://github.com/linusg

Userland/Libraries/LibGUI/Forward.h

@@ -23,6 +23,7 @@ class CheckBox;
 class ComboBox;
 class Command;
 class CommandPalette;
+class Dialog;
 class DialogButton;
 class DragEvent;
 class DropEvent;