block: move blk_throtl prototypes to block/blk.h

blk_throtl interface is block internal and there's no reason to have
them in linux/blkdev.h.  Move them to block/blk.h.

This patch doesn't introduce any functional change.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index a19f58c..f3f495e 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -10,6 +10,7 @@
 #include <linux/bio.h>
 #include <linux/blktrace_api.h>
 #include "blk-cgroup.h"
+#include "blk.h"
 
 /* Max dispatch from a group in 1 round */
 static int throtl_grp_quantum = 8;