add: 补充luat_pm_get_poweron_reason的头文件声明

luat/include/luat_pm.h

@@ -42,4 +42,7 @@ int luat_pm_dtimer_wakeup_id(int* id);
 int luat_pm_poweroff(void);
 
 int luat_pm_power_ctrl(int id, uint8_t onoff);
+
+int luat_pm_get_poweron_reason(void);
+
 #endif