Add license header to "AdaptiveColumnsPanel.cs"

Author: giovanni-a

src/OpenSilver.ControlsKit.Controls/AdaptiveColumnsPanel.cs

@@ -1,4 +1,16 @@
-using System;
+/*===================================================================================
+*
+*   Copyright (c) Userware (OpenSilver.net)
+*
+*   This file is part of the OpenSilver.ControlsKit (https://opensilver.net), which
+*   is licensed under the MIT license (https://opensource.org/licenses/MIT).
+*
+*   As stated in the MIT license, "the above copyright notice and this permission
+*   notice shall be included in all copies or substantial portions of the Software."
+*
+*====================================================================================*/
+
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Windows;