[NaCl SDK] Enable linux host build for nacl_io and nacl_io_tests.

native_client_sdk/src/libraries/nacl_io/fifo_null.h

 // Copyright (c) 2013 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 LIBRARIES_NACL_IO_FIFO_NULL_H_
 #define LIBRARIES_NACL_IO_FIFO_NULL_H_
 
 #include <stdint.h>
 #include <stdlib.h>
 
 namespace nacl_io {
 
 // FIFONull
 //
 // A null fifo is always ready to read or write, but never actually
 // provides or stores data.
 class FIFONull : public FIFOInterface {
  public:
   virtual bool IsEmpty() { return false; }
   virtual bool IsFull() { return false; }
   virtual bool Resize(size_t) { return false; }
 
-  virtual uint32_t ReadAvailable() { return 1; }
-  virtual uint32_t WriteAvailable() { return 1; }
+  virtual size_t ReadAvailable() { return 1; }
+  virtual size_t WriteAvailable() { return 1; }
 };
 
 }  // namespace nacl_io
 
 #endif  // LIBRARIES_NACL_IO_FIFO_NULL_H_