um: set BLK_CGROUP=y in defconfig

BLK_CGROUP=m is no longer valid.
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/defconfig

@@ -163,7 +163,7 @@ CONFIG_CGROUP_SCHED=y
 CONFIG_FAIR_GROUP_SCHED=y
 # CONFIG_CFS_BANDWIDTH is not set
 # CONFIG_RT_GROUP_SCHED is not set
-CONFIG_BLK_CGROUP=m
+CONFIG_BLK_CGROUP=y
 # CONFIG_DEBUG_BLK_CGROUP is not set
 # CONFIG_CHECKPOINT_RESTORE is not set
 CONFIG_NAMESPACES=y