Fix missing using after merge conflict

Author: h3xds1nz

src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlSchemaContext.cs

@@ -8,6 +8,7 @@
 using System.Collections.ObjectModel;
 using System.Reflection;
 using System.Text;
+using MS.Internal;
 using System.Threading;
 using System.Xaml.MS.Impl;
 using System.Xaml.Schema;