Only activate __EXT1_VISIBLE block when using sys/errno.h in userspace.

The prerequisite for '#if __EXT1_VISIBLE' functionality is the
inclusion of sys/cdefs.h. errno.h only auto-includes the header for
non-kernel environment, and EXT1 block only useful for non-kernel as
well.

Reported by: lwhsu
Sponsored by: The FreeBSD Foundation
MFC after: 3 weeks