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: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>