block: add SPDX header line to blk-throttle.h

Add an SPDX license identifier line to blk-throttle.h

Use 'GPL-2.0' as the identifier, since blk-throttle.c uses
that, and blk.h (from which some material was copied when
blk-throttle.h was created) also uses that identifier.

Signed-off-by: Tim Bird <tim.bird@sony.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/MW5PR13MB5632EE4645BCA24ED111EC0EFDB62@MW5PR13MB5632.namprd13.prod.outlook.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>