Kernel headers shouldn't expose functions to userspace.

Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>