vhost: Remove the repeated declaration

Function 'vhost_vring_ioctl' is declared twice, remove the repeated
declaration.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Link: https://lore.kernel.org/r/1621857884-19964-1-git-send-email-zhangshaokun@hisilicon.com
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>