vhost_vdpa: Fix duplicate included kernel.h

linux/kernel.h is included more than once, Remove the one that isn't
necessary.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Link: https://lore.kernel.org/r/1600131102-24672-1-git-send-email-tiantao6@hisilicon.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>