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>
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 5b9c6d5..b231935 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -9,7 +9,7 @@
 #include <linux/blkdev.h>
 #include <linux/bio.h>
 #include <linux/blktrace_api.h>
-#include "blk-cgroup.h"
+#include <linux/blk-cgroup.h>
 #include "blk.h"
 
 /* Max dispatch from a group in 1 round */