Although the kernel doesn't use this, qemu imports these headers
and it's best to keep them consistent.

This define is also something userspace may want to use.
Signed-off-by: NDave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180503021021.10694-1-airlied@gmail.comSigned-off-by: NGerd Hoffmann <kraxel@redhat.com>