nvme: fabrics drivers don't need the nvme-pci driver

So select the NVME_CORE symbol instead of depending on BLK_DEV_NVME.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jay Freyensee <james_p_freyensee@linux.intel.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>