sched, trivial: Initialize root cgroup's sibling list

Even though there are no siblings, the list should be
initialized to not contain bogus values.

Signed-off-by: Glauber Costa <[e-mail]>
Acked-by: Paul Menage <[e-mail]>
Acked-by: Paul Turner <[e-mail]>
Signed-off-by: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/sched.c

···
 #ifdef CONFIG_CGROUP_SCHED
 	list_add(&root_task_group.list, &task_groups);
 	INIT_LIST_HEAD(&root_task_group.children);
+	INIT_LIST_HEAD(&root_task_group.siblings);
 	autogroup_init(&init_task);
 #endif /* CONFIG_CGROUP_SCHED */