esp8266: Protect modpyb.h header file from multiple inclusions.

Also include py/obj.h so the header is self contained.

esp8266/modpyb.h

+#ifndef __MICROPY_INCLUDED_ESP8266_MODPYB_H__
+#define __MICROPY_INCLUDED_ESP8266_MODPYB_H__
+
+#include "py/obj.h"
+
 extern const mp_obj_type_t pyb_pin_type;
 extern const mp_obj_type_t pyb_pwm_type;
 extern const mp_obj_type_t pyb_adc_type;
@@ -18,3 +23,5 @@ uint mp_obj_get_pin(mp_obj_t pin_in);
 pyb_pin_obj_t *mp_obj_get_pin_obj(mp_obj_t pin_in);
 int pin_get(uint pin);
 void pin_set(uint pin, int value);
+
+#endif // __MICROPY_INCLUDED_ESP8266_MODPYB_H__