sched_ext: Always use SMP versions in kernel/sched/ext.h

Simplify the scheduler by making formerly SMP-only primitives and data
structures unconditional.

tj: Updated subject for clarity. Replace #if defined() with #ifdef.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: EricccTaiwan

kernel/sched/ext.h

@@ -81,7 +81,7 @@ static inline void init_sched_ext_class(void) {}
 
 #endif	/* CONFIG_SCHED_CLASS_EXT */
 
-#if defined(CONFIG_SCHED_CLASS_EXT) && defined(CONFIG_SMP)
+#ifdef CONFIG_SCHED_CLASS_EXT
 void __scx_update_idle(struct rq *rq, bool idle, bool do_notify);
 
 static inline void scx_update_idle(struct rq *rq, bool idle, bool do_notify)