userfaultfd: remove kernel header include from uapi header

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 <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>