sched/fair: Define sched_idle_cpu() only for SMP configurations

sched_idle_cpu() isn't used for non SMP configuration and with a recent
change, we have started getting following warning:

kernel/sched/fair.c:5221:12: warning: ‘sched_idle_cpu’ defined but not used [-Wunused-function]

Fix that by defining sched_idle_cpu() only for SMP configurations.

Fixes: 323af6deaf70 ("sched/fair: Load balance aggressively for SCHED_IDLE CPUs")
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Viresh Kumar <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Juri Lelli <[e-mail]>
Cc: Vincent Guittot <[e-mail]>
Cc: Dietmar Eggemann <[e-mail]>
Link: https://lore.kernel.org/r/f055[e-mail]

kernel/sched/fair.c

···
 			rq->nr_running);
 }
 
+#ifdef CONFIG_SMP
 static int sched_idle_cpu(int cpu)
 {
 	return sched_idle_rq(cpu_rq(cpu));
 }
+#endif
 
 /*
  * The enqueue_task method is called before nr_running is