C#: Ensure bi-directional import for External flow.

csharp/ql/lib/semmle/code/csharp/dataflow/ExternalFlow.qll

@@ -97,6 +97,7 @@ private module Frameworks {
   private import semmle.code.csharp.frameworks.system.Web
   private import semmle.code.csharp.frameworks.system.collections.Generic
   private import semmle.code.csharp.frameworks.system.web.ui.WebControls
+  private import semmle.code.csharp.frameworks.JsonNET
 }
 
 /**