Make _cyclic_ timers interval more deterministic - next timeout is calculated from last due time instead of relative to current time

This eliminates the cyclic timer jitter

src/include/lwip/timeouts.h

@@ -116,6 +116,7 @@ void sys_timeouts_mbox_fetch(sys_mbox_t *mbox, void **msg);
 
 #if LWIP_TESTMODE
 struct sys_timeo** lwip_sys_timers_get_next_timout(void);
+void lwip_cyclic_timer(void *arg);
 #endif
 
 #endif /* LWIP_TIMERS */