sched/headers: Move the wake_up_if_idle() prototype to <linux/sched/idle.h>

No need to clutter <linux/sched.h> with this rarely used prototype.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched/idle.h

@@ -10,4 +10,6 @@ enum cpu_idle_type {
 	CPU_MAX_IDLE_TYPES
 };
 
+extern void wake_up_if_idle(int cpu);
+
 #endif /* _LINUX_SCHED_IDLE_H */