Declare virtual destructor in Nacl plugin CallbackSource to fix mac build

ppapi/native_client/src/trusted/plugin/callback_source.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_CALLBACK_SOURCE_H
 #define NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_CALLBACK_SOURCE_H
 
 #include "ppapi/cpp/completion_callback.h"
 
 namespace plugin {
 
 // Classes that implement this interface can be used with FileDownloader's
 // DOWNLOAD_STREAM mode. For each chunk of the file as it downloads,
 // The FileDownloader will get a callback of type
 // pp::CompletionCallbackWithOutput<std::vector<char>*> using the
 // GetCallback function, and call it immediately, passing a pointer to a
 // vector with the data as the output field. The class is templatized just
 // in case there are any other use cases in the future.
 // This class really only exists as a way to get callbacks
 // bound to an object (i.e. we don't need the asynchronous behavior of PPAPI)
 // All we really need is tr1::function or base::bind or some such, but we don't
 // have those in the plugin.
 
 template<class T>
 class CallbackSource {
  public:
+  virtual ~CallbackSource();
   // Returns a callback from callback_factory's NewCallbackWithOutput,
   // bound to the implementing object.
   virtual pp::CompletionCallbackWithOutput<T> GetCallback() = 0;
 };
 }
 #endif // NATIVE_CLIENT_SRC_TRUSTED_PLUGIN_CALLBACK_SOURCE_H