linux/hdsmart.h: fix goofups (take 2)

Fix goofups of commit 76166952
("<linux/hdsmart.h> is not used by kernel code").

Also update include/linux/Kbuild to reflect the fact that hdsmart.h
uses __KERNEL__ ifdefs now.

Reported-by: "Robert P. J. Day" <rpjday@crashcourse.ca>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>