py/mphal: Add stddef.h header for size_t.

This includes "stddef.h" for `size_t` to resolve NuttX integration build
issues.

Signed-off-by: Yanfeng Liu <[email protected]>

Author: yf13

py/mphal.h

@@ -27,6 +27,7 @@
 #define MICROPY_INCLUDED_PY_MPHAL_H
 
 #include <stdint.h>
+#include <stddef.h>
 #include "py/mpconfig.h"
 
 #ifdef MICROPY_MPHALPORT_H