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 <firstname.lastname@example.org>
Signed-off-by: Christoph Hellwig <email@example.com>
Signed-off-by: Jens Axboe <firstname.lastname@example.org>