io controller: quick fix for blk-cgroup and modular CFQ

It's currently not an allowed configuration, so express that in Kconfig.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/block/Kconfig.iosched b/block/Kconfig.iosched
index b71abfb..5368d74 100644
--- a/block/Kconfig.iosched
+++ b/block/Kconfig.iosched
@@ -34,7 +34,7 @@
 
 config CFQ_GROUP_IOSCHED
 	bool "CFQ Group Scheduling support"
-	depends on IOSCHED_CFQ && CGROUPS
+	depends on IOSCHED_CFQ=y && CGROUPS
 	select BLK_CGROUP
 	default n
 	---help---