[NaCl SDK] Add initial support for building the SDK with gn

ppapi/native_client/src/untrusted/irt_stub/ppapi_plugin_main.c

 /*
  * Copyright (c) 2011 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.
  */
 
+#include "ppapi/native_client/src/shared/ppapi_proxy/ppruntime.h"
+
 #include "ppapi/native_client/src/untrusted/irt_stub/ppapi_start.h"
 
 /*
  * These are dangling references to functions that the application must define.
  */
 static const struct PP_StartFunctions ppapi_app_start_callbacks = {
   PPP_InitializeModule,
   PPP_ShutdownModule,
   PPP_GetInterface
 };
 
 /*
  * The application's main (or the one supplied in this library) calls this
  * to start the PPAPI world.
  */
 int PpapiPluginMain(void) {
   return PpapiPluginStart(&ppapi_app_start_callbacks);
 }