Add missing license header

Author: robloo

Microsoft.Toolkit.Uwp.UI.Controls/ColorPicker/ColorPickerSlider.Properties.cs

@@ -1,4 +1,8 @@
-using Microsoft.Toolkit.Uwp.Helpers;
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.Toolkit.Uwp.Helpers;
 using Windows.UI;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;