Add additional include guard in platform specific .h files

runtime/bin/thread_pool_linux.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef BIN_THREAD_POOL_LINUX_H_
 #define BIN_THREAD_POOL_LINUX_H_
 
+#if !defined(BIN_THREAD_POOL_H_)
+#error Do not include thread_pool_linux.h directly; use thread_pool.h instead.
+#endif
+
 #include <pthread.h>
 
 #include "platform/globals.h"
 
 
 class ThreadPoolData {
  private:
   ThreadPoolData() {}
   ~ThreadPoolData() {}
 
   pthread_t* threads() { return threads_; }
   void set_threads(pthread_t* threads) { threads_ = threads; }
 
   pthread_t* threads_;
 
   friend class ThreadPool;
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(ThreadPoolData);
 };
 
 #endif  // BIN_THREAD_POOL_LINUX_H_