block: add a SPDX tag to blk-mq-rdma.h

This file has no copyright notice, but was added as part of a commit
adding another file using the default kernel GPLv2 license.  Add
a matching SPDX tag.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>