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>

include/linux/writeback.h

@@ -336,14 +336,9 @@ static inline void cgroup_writeback_umount(void)
 /*
  * mm/page-writeback.c
  */
-#ifdef CONFIG_BLOCK
 void laptop_io_completion(struct backing_dev_info *info);
 void laptop_sync_completion(void);
-void laptop_mode_sync(struct work_struct *work);
 void laptop_mode_timer_fn(struct timer_list *t);
-#else
-static inline void laptop_sync_completion(void) { }
-#endif
 bool node_dirty_ok(struct pglist_data *pgdat);
 int wb_domain_init(struct wb_domain *dom, gfp_t gfp);
 #ifdef CONFIG_CGROUP_WRITEBACK