Add the missing file.

src/Markup/Avalonia.Markup.Xaml/XamlLoadException.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace Avalonia.Markup.Xaml
+{
+    public class XamlLoadException: Exception
+    {
+        public XamlLoadException()
+        {
+        }
+
+        protected XamlLoadException(SerializationInfo info, StreamingContext context): base(info, context)
+        {
+        }
+
+        public XamlLoadException(string message): base(message)
+        {
+        }
+
+        public XamlLoadException(string message, Exception innerException): base(message, innerException)
+        {
+        }
+    }
+}