writeback: make the laptop_mode prototypes available unconditionally

Fix the !CONFIG_BLOCK build after the recent cleanup.

Fixes: 5ed964f8 ("mm: hide laptop_mode_wb_timer entirely behind the BDI API")
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>