include/uapi/linux/dn.h: pull in ioctl.h header

This header uses _IOW/_IOR defines but doesn't include ioctl.h for it.
If you try to use this w/out including ioctl.h yourself, it can fail to
build, so add the explicit include.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>