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: 323af6de ("sched/fair: Load balance aggressively for SCHED_IDLE CPUs")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/f0554f590687478b33914a4aff9f0e6a62886d44.1579499907.git.viresh.kumar@linaro.org

kernel/sched/fair.c

@@ -5218,10 +5218,12 @@ static int sched_idle_rq(struct rq *rq)
 			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