pm_runtime: fix missing header

if the sched.h not included by header already inside .c will cause
sched_idletask not defined

Signed-off-by: buxiasen <buxiasen@xiaomi.com>

drivers/power/pm/pm_runtime.c

@@ -27,6 +27,7 @@
 #include <debug.h>
 #include <assert.h>
 #include <errno.h>
+#include <sched.h>
 #include <nuttx/clock.h>
 #include <nuttx/power/pm_runtime.h>
 #include <sched/sched.h>