blkcg: blk-iocost: predeclare used structs

Fixes: 7caa4715 ("blkcg: implement blk-iocost")
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/trace/events/iocost.h

@@ -2,6 +2,10 @@
 #undef TRACE_SYSTEM
 #define TRACE_SYSTEM iocost
 
+struct ioc;
+struct ioc_now;
+struct ioc_gq;
+
 #if !defined(_TRACE_BLK_IOCOST_H) || defined(TRACE_HEADER_MULTI_READ)
 #define _TRACE_BLK_IOCOST_H