sched_ext: Allow dequeue_task_scx to fail

Since dequeue_task() allowed to fail, there is a compile error:

kernel/sched/ext.c:3630:19: error: initialization of ‘bool (*)(struct rq*, struct task_struct *, int)’ {aka ‘_Bool (*)(struct rq *, struct task_struct *, int)’} from incompatible pointer type ‘void (*)(struct rq*, struct task_struct *, int)’
  3630 |  .dequeue_task  = dequeue_task_scx,
       |                   ^~~~~~~~~~~~~~~~

Allow dequeue_task_scx to fail too.

Fixes: 863ccdbb ("sched: Allow sched_class::dequeue_task() to fail")
Signed-off-by: Yipeng Zou <zouyipeng@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>