timerfd: export defines to userspace

Since userspace is expected to call timerfd syscalls directly with these
flags/ioctls, make sure we export them so they don't have to duplicate
the values themselves.

Link: http://lkml.kernel.org/r/20161219064052.7196-1-vapier@gentoo.org
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>