blkcg: move block/blk-cgroup.h to include/linux/blk-cgroup.h

cgroup aware writeback support will require exposing some of blkcg
details.  In preprataion, move block/blk-cgroup.h to
include/linux/blk-cgroup.h.  This patch is pure file move.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from commit eea8f41cc58849e354ecf8b95bd7f806e1d1f703)
Signed-off-by: Alex Shi <alex.shi@linaro.org>
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 2b8fd30..58991dd 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -8,9 +8,9 @@
 #include <linux/blkdev.h>
 #include <linux/blktrace_api.h>
 #include <linux/blk-mq.h>
+#include <linux/blk-cgroup.h>
 
 #include "blk.h"
-#include "blk-cgroup.h"
 #include "blk-mq.h"
 
 struct queue_sysfs_entry {