Update enabledisable.cs

Author: davmason

src/tests/tracing/eventpipe/enabledisable/enabledisable.cs

@@ -5,6 +5,7 @@
 using Microsoft.Diagnostics.Tracing;
 using Microsoft.Diagnostics.Tracing.Parsers.Clr;
 using System;
+using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Diagnostics.Tracing;
 using System.IO;