virtio_blk: add a missing include

virtio_blk uses VIRTIO_RING_F_INDIRECT_DESC, pull in
the header defining that value.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/block/virtio_blk.c

@@ -15,6 +15,7 @@
 #include <linux/blk-mq.h>
 #include <linux/blk-mq-virtio.h>
 #include <linux/numa.h>
+#include <uapi/linux/virtio_ring.h>
 
 #define PART_BITS 4
 #define VQ_NAME_LEN 16