demo: fixed compile

demo/ButtonGenerator/DemoButtonGenerator.pas

@@ -5,7 +5,8 @@ unit DemoButtonGenerator;
 interface
 
 uses
-  Classes, SysUtils, Fresnel.DOM, Fresnel.Controls, DemoSlider, DemoCheckBox;
+  Classes, SysUtils, Fresnel.DOM, Fresnel.Controls, Fresnel.Classes, DemoSlider,
+  DemoCheckBox;
 
 type