cpp: Make an implicit virtual declaration explicit

This changes nothing since it's already declared virtual in the parent,
but this is clearer.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@920690 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/cpp/src/transport/TSocket.h b/lib/cpp/src/transport/TSocket.h
index b9d1c01..70c167f 100644
--- a/lib/cpp/src/transport/TSocket.h
+++ b/lib/cpp/src/transport/TSocket.h
@@ -86,7 +86,7 @@
   /**
    * Shuts down communications on the socket.
    */
-  void close();
+  virtual void close();
 
   /**
    * Reads from the underlying socket.