C#: Add missing bi-directional imports.

Author: michaelnebel

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

@@ -102,6 +102,7 @@ private module Frameworks {
   private import semmle.code.csharp.frameworks.ServiceStack
   private import semmle.code.csharp.frameworks.Sql
   private import semmle.code.csharp.frameworks.System
+  private import semmle.code.csharp.frameworks.system.CodeDom
   private import semmle.code.csharp.frameworks.system.Collections
   private import semmle.code.csharp.frameworks.system.collections.Concurrent
   private import semmle.code.csharp.frameworks.system.collections.Generic
@@ -122,6 +123,7 @@ private module Frameworks {
   private import semmle.code.csharp.frameworks.system.IO
   private import semmle.code.csharp.frameworks.system.io.Compression
   private import semmle.code.csharp.frameworks.system.runtime.CompilerServices
+  private import semmle.code.csharp.frameworks.system.Security
   private import semmle.code.csharp.frameworks.system.security.Cryptography
   private import semmle.code.csharp.frameworks.system.security.cryptography.X509Certificates
   private import semmle.code.csharp.frameworks.system.Text