skein: Update guard define check

OpenZFS changed the define _OPENSOLARIS_SYS_TYPES_H_ to
_SPL_SYS_TYPES_H_ to guard the sys/types.h compatibility file
inclusion. Follow the change here. The only place in the tree
_OPENSOLARIS_SYS_TYPES_H_ is mentioned is in the /*
_OPENSOLARIS_SYS_TYPES_H_ */ at the end of sys/types.h. That needs to be
changed upstream in OpenZFS since we don't like changing things in
FreeBSD's tree.

Sponsored by: Netflix
Reviewed by: tsoome, delphij
Differential Revision: https://reviews.freebsd.org/D35891