Fix issue building in arm-none-eabi-gcc v6.

Including stdio.h before MicroBit.h caused a stdio macro for
putc(int, FILE*) to cause havoc on putc methods in multiple CODAL
classes like serial.

source/samples/MicrophoneTest.cpp

@@ -1,10 +1,10 @@
-#include <stdio.h>
 #include "MicroBit.h"
 #include "SerialStreamer.h"
 #include "StreamNormalizer.h"
 #include "LevelDetector.h"
 #include "LevelDetectorSPL.h"
 #include "Tests.h"
+#include <stdio.h>
 
 static NRF52ADCChannel *mic = NULL;
 static SerialStreamer *streamer = NULL;