circuitpy_mpconfig.h: Move includes after include-guard

To benefit from gcc's "once-only headers" implementation, the
"wrapper-#ifndef" must be the first non-comment part of the file,
according to the manual for various gcc/cpp versions.

py/circuitpy_mpconfig.h

@@ -28,11 +28,11 @@
 // sure that the same feature set and settings are used, such as in atmel-samd
 // and nrf.
 
-#include <stdint.h>
-
 #ifndef __INCLUDED_MPCONFIG_CIRCUITPY_H
 #define __INCLUDED_MPCONFIG_CIRCUITPY_H
 
+#include <stdint.h>
+
 // This is CircuitPython.
 #define CIRCUITPY 1