introduce sys_timeouts_sleeptime (returns the time left before the next timeout is due, for NO_SYS==1)

src/include/lwip/timers.h

@@ -87,6 +87,7 @@ void sys_untimeout(sys_timeout_handler handler, void *arg);
 #if NO_SYS
 void sys_check_timeouts(void);
 void sys_restart_timeouts(void);
+u32_t sys_timeouts_sleeptime(void);
 #else /* NO_SYS */
 void sys_timeouts_mbox_fetch(sys_mbox_t *mbox, void **msg);
 #endif /* NO_SYS */