Fix libc printf on non-AVR for Arduino core 1.8.x

With debugging enabled, LMIC wouldn't compile on SAMD targets when used with Arduino core greater than 1.6.21 (eg 1.8.x versions). Changing the order of includes fixes this. Suggested by matthijskooiman in upstream matthijskooijman#270.

src/hal/hal.cpp

@@ -8,13 +8,13 @@
  * This the HAL to run LMIC on top of the Arduino environment.
  *******************************************************************************/
 
+#define _GNU_SOURCE 1 // For fopencookie
+#include <stdio.h>
+#undef _GNU_SOURCE
 #include <Arduino.h>
 #include <SPI.h>
 #include "../lmic.h"
 #include "hal.h"
-#define _GNU_SOURCE 1 // For fopencookie
-#include <stdio.h>
-#undef _GNU_SOURCE
 
 // -----------------------------------------------------------------------------
 // I/O