Include stddef.h. It is required by size_t. In case you include stdio.h,...

Include stddef.h. It is required by size_t. In case you include stdio.h, stddef.h is included indirectly.

include/mrbconf.h

@@ -8,6 +8,7 @@
 #define MRUBYCONF_H
 
 #include <stdint.h>
+#include <stddef.h>
 
 /* configuration options: */
 /* add -DMRB_USE_FLOAT to use float instead of double for floating point numbers */