Fix ProjectN build breaks (dotnet/corert#7115)

Signed-off-by: dotnet-bot <anirudhagnihotry098@gmail.com>

Author: MichalStrehovsky

src/System.Private.CoreLib/shared/System/Environment.WinRT.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.IO;
+using Internal.Runtime.Augments;
 
 namespace System
 {