esp8266: Fix ticks_ms to correctly handle wraparound of system counter.

Fixes issue #4795.
diff --git a/ports/esp8266/ets_alt_task.h b/ports/esp8266/ets_alt_task.h
index e7a15c3..7eb8ff3 100644
--- a/ports/esp8266/ets_alt_task.h
+++ b/ports/esp8266/ets_alt_task.h
@@ -3,8 +3,10 @@
 
 extern int ets_loop_iter_disable;
 extern int ets_loop_dont_feed_sw_wdt;
+extern uint32_t system_time_low_word;
 extern uint32_t system_time_high_word;
 
+void system_time_update(void);
 bool ets_loop_iter(void);
 
 #endif // MICROPY_INCLUDED_ESP8266_ETS_ALT_TASK_H