Update license header.

src/Avalonia.Base/Collections/Pooled/ClearMode.cs

@@ -1,4 +1,7 @@
-namespace Avalonia.Collections.Pooled
+// This source file is adapted from the Collections.Pooled.
+// (https://github.com/jtmueller/Collections.Pooled/tree/master/Collections.Pooled/)
+
+namespace Avalonia.Collections.Pooled
 {
     /// <summary>
     /// This enum allows control over how data is treated when internal

src/Avalonia.Base/Collections/Pooled/IReadOnlyPooledList.cs

@@ -1,4 +1,7 @@
-using System;
+// This source file is adapted from the Collections.Pooled.
+// (https://github.com/jtmueller/Collections.Pooled/tree/master/Collections.Pooled/)
+
+using System;
 using System.Collections.Generic;
 
 namespace Avalonia.Collections.Pooled