cgroup: export css_next_descendant_pre for bfq

The bfq schedule now uses css_next_descendant_pre directly after
the stats functionality depending on it has been from the core
blk-cgroup code to bfq.  Export the symbol so that bfq can still
be build modular.

Fixes: d625898 ("bfq-iosched: move bfq_stat_recursive_sum into the only caller")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/cgroup/cgroup.c

@@ -4221,6 +4221,7 @@ css_next_descendant_pre(struct cgroup_subsys_state *pos,
 
 	return NULL;
 }
+EXPORT_SYMBOL_GPL(css_next_descendant_pre);
 
 /**
  * css_rightmost_descendant - return the rightmost descendant of a css