btrfs: add btrfs_delayed_ref_head declaration to extent-tree.h

extent-tree.h uses btrfs_delayed_ref_head in a function argument but
doesn't pull it's declaration from anywhere, add it to the top of the
header.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.h

@@ -7,6 +7,7 @@
 #include "block-group.h"
 
 struct btrfs_free_cluster;
+struct btrfs_delayed_ref_head;
 
 enum btrfs_extent_allocation_policy {
 	BTRFS_EXTENT_ALLOC_CLUSTERED,