block: Make blk_rq_set_mixed_merge() static

Since commit 8e756373

 ("block: Move bio merge related functions into
blk-merge.c"), blk_rq_set_mixed_merge() has only been referenced in
blk-merge.c, so make it static.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240325083501.2816408-1-john.g.garry@oracle.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>