Fix ImplementedFunction serialization error

Author: Perksey

src/Core/Silk.NET.BuildTools/Common/ImplementedFunction.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Text;
@@ -13,6 +14,12 @@ namespace Silk.NET.BuildTools.Common
     /// </summary>
     public class ImplementedFunction
     {
+        /// <summary>
+        /// Do not use, for serialization only.
+        /// </summary>
+        [Obsolete("Do not use, for serialization only.", true)]
+        public ImplementedFunction() {}
+        
         /// <summary>
         /// Creates a new ImplementedFunction.
         /// </summary>