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: NCheng Jian <cj.chengjian@huawei.com>
Reviewed-by: NBin Li <huawei.libin@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

kernel/sched/fair.c

@@ -3901,6 +3901,7 @@ static inline int idle_balance(struct rq *rq, struct rq_flags *rf)
 
 static inline void rq_idle_stamp_update(struct rq *rq) {}
 static inline void rq_idle_stamp_clear(struct rq *rq) {}
+static inline int try_steal(struct rq *this_rq, struct rq_flags *rf) { return 0; }
 static inline void overload_clear(struct rq *rq) {}
 static inline void overload_set(struct rq *rq) {}