nvme-rdma: add back dependency on CONFIG_BLOCK

A recent change removed the dependency on BLK_DEV_NVME, which implies
the dependency on PCI and BLOCK. We don't need CONFIG_PCI, but without
CONFIG_BLOCK we get tons of build errors, e.g.

In file included from drivers/nvme/host/core.c:16:0:
linux/blk-mq.h:182:33: error: 'struct gendisk' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]
drivers/nvme/host/core.c: In function 'nvme_setup_rw':
drivers/nvme/host/core.c:295:21: error: implicit declaration of function 'rq_data_dir' [-Werror=implicit-function-declaration]
drivers/nvme/host/nvme.h: In function 'nvme_map_len':
drivers/nvme/host/nvme.h:217:6: error: implicit declaration of function 'req_op' [-Werror=implicit-function-declaration]
drivers/nvme/host/scsi.c: In function 'nvme_trans_bdev_limits_page':
drivers/nvme/host/scsi.c:768:85: error: implicit declaration of function 'queue_max_hw_sectors' [-Werror=implicit-function-declaration]

This adds back the specific CONFIG_BLOCK dependency to avoid broken
configurations.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: aa719874 ("nvme: fabrics drivers don't need the nvme-pci driver")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>