Fix compilation error when Esp.h is included first in sketch (esp8266#590)

Author: igrr

hardware/esp8266com/esp8266/cores/esp8266/Esp.h

@@ -1,8 +1,8 @@
-/* 
+/*
  Esp.h - ESP8266-specific APIs
  Copyright (c) 2015 Ivan Grokhotkov. All rights reserved.
  This file is part of the esp8266 core for Arduino environment.
- 
+
  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
@@ -20,6 +20,9 @@
 
 #ifndef ESP_H
 #define ESP_H
+
+#include <Arduino.h>
+
 /**
  * AVR macros for WDT managment
  */