um: Remove copy&paste code from init.h

commit 30b11ee9ae23d78de66b9ae315880af17a64ba83 upstream.

As we got rid of the __KERNEL__ abuse, we can directly
include linux/compiler.h now.
This also allows gcc 5 to build UML.

Reported-by: Hans-Werner Hilse <hwhilse@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Bernie Innocenti <codewiz@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Tkkg1994 <luca@grifodev.ch>