virtio: Fix VirtIO BLK driver dependency

The VirtIO BLK driver depends on the blk uclass driver.
Add the dependency in the Kconfig.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index e800720..1835607 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -55,6 +55,7 @@
 config VIRTIO_BLK
 	bool "virtio block driver"
 	depends on VIRTIO
+	depends on BLK
 	help
 	  This is the virtual block driver for virtio. It can be used with
 	  QEMU based targets.