Use stdlib.h to get size_t.

#ifndef HEADER_RAND_H

#define HEADER_RAND_H

#include <openssl/e_os2.h>

#if defined(OPENSSL_SYS_WINDOWS) || defined(OPENSSL_SYS_WIN32)

#include <windows.h>

#else

#include <sys/types.h>

#endif

#include <stdlib.h>

#ifdef  __cplusplus

extern "C" {