Don't include <stddef.h>

Including a standard or system header isn't allowed in OpenCL.

The type "size_t" needs to be explicitely defined now.

v2: Use __SIZE_TYPE__ instead of unsigned int.
v3: Define ptrdiff_t and NULL.

Patch-by: Jean-Sébastien Pédron
Reviewed-by: Jeroen Ketema
Reviewed-by: Jan Vesely