Add missing using declaration

Author: pavel-alay

itext/itext.kernel/KernelExtensions.cs

@@ -51,6 +51,7 @@ source product.
 using System.Text;
 using System.Text.RegularExpressions;
 using iText.IO.Util;
+using iText.IO.Util.Collections;
 using Org.BouncyCastle.Crypto;
 
 internal static class KernelExtensions {