Remove stdlib.h from mruby.h. It is for portability (care for freestanding environments).

This is a first step. It will be reduced stdlib.h in each files later.

src/string.c

@@ -7,6 +7,7 @@
 #include "mruby.h"
 
 #include <stddef.h>
+#include <stdlib.h>
 #include <string.h>
 #include "mruby/string.h"
 #include "mruby/class.h"