sched/fair: fix try_steal compile error

hulk inclusion
category: bugfix
bugzilla: 38261, https://bugzilla.openeuler.org/show_bug.cgi?id=23


CVE: NA

---------------------------

If we disable CONFIG_SMP, try_steal will lose its definition,
resulting in a compile error as follows.

	kernel/sched/fair.c: In function ‘pick_next_task_fair’:
	kernel/sched/fair.c:7001:15: error: implicit declaration of function ‘try_steal’ [-Werror=implicit-function-declaration]
		new_tasks = try_steal(rq, rf);
			    ^~~~~~~~~

We can use allnoconfig to reproduce this problem.

Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Bin Li <huawei.libin@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>