userfaultfd: remove kernel header include from uapi header

As include/uapi/linux/userfaultfd.h is a user visible header file, it
should not include kernel-exclusive header files.

So trying to build the userfaultfd test program from the selftests
directory fails, since it contains a reference to linux/compiler.h.  As
it turns out, that header is not really needed there, so we can simply
remove it to fix that issue.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>