Fix unknown type name "size_t" error

This commit adds an include clause to "boot_record.h" to include the stddef.h header that contains the declaration of the "size_t" type.

Attempting to include "boot_record.h" in a build for Mbed-OS would throw an error complaining "size_t" was unknown, adding a note to inculde "stddef.h"

Signed-off-by: George Beckstein <george.beckstein@gmail.com>