NVMe: Add explicit block config dependency

The nvme driver was moved from drivers/block, losing our implicit
dependency on CONFIG_BLOCK. This makes it an explicit driver dependency.
Reported-by: NJim Davis <jim.epost@gmail.com>
Signed-off-by: NKeith Busch <keith.busch@intel.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

drivers/nvme/host/Kconfig

 config BLK_DEV_NVME
 	tristate "NVM Express block device"
-	depends on PCI
+	depends on PCI && BLOCK
 	---help---
 	  The NVM Express driver is for solid state drives directly
 	  connected to the PCI or PCI Express bus.  If you know you