block: fix a warning when blkdev.h is included for !CONFIG_BLOCK builds

disk_start_io_acct and disk_end_io_acct need at least a struct gendisk
forward declaration, but for weird historic reasons much of blkdev.h
is stubbed out for CONFIG_BLOCK=n.  Fix this by stubbing more out for
now, but eventually this header will need a massive cleanup.

Fixes: 956d510e ("block: add disk/bio-based accounting helpers")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -1892,12 +1892,12 @@ static inline void blk_wake_io_task(struct task_struct *waiter)
 		wake_up_process(waiter);
 }
 
+#ifdef CONFIG_BLOCK
 unsigned long disk_start_io_acct(struct gendisk *disk, unsigned int sectors,
 		unsigned int op);
 void disk_end_io_acct(struct gendisk *disk, unsigned int op,
 		unsigned long start_time);
 
-#ifdef CONFIG_BLOCK
 /**
  * bio_start_io_acct - start I/O accounting for bio based drivers
  * @bio:	bio to start account for