Fix public -> private in ui::Timer

src/ui/LICENSE.txt

@@ -1,4 +1,4 @@
-Copyright (c) 2018-2020 Igara Studio S.A.
+Copyright (c) 2018-2022 Igara Studio S.A.
 Copyright (c) 2001-2018 David Capello
 
 Permission is hereby granted, free of charge, to any person obtaining

src/ui/timer.h

@@ -1,5 +1,5 @@
 // Aseprite UI Library
-// Copyright (C) 2020-2021  Igara Studio S.A.
+// Copyright (C) 2020-2022  Igara Studio S.A.
 // Copyright (C) 2001-2016  David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -43,7 +43,7 @@ namespace ui {
   protected:
     virtual void onTick();
 
-  public:
+  private:
     Widget* m_owner;
     int m_interval;
     bool m_running;