esp8266: Extend system microsecond counter to 64-bits; use in ticks_ms.

So now ticks_ms can count up to the full 30 bits.  Fixes issue #2412.
diff --git a/esp8266/ets_alt_task.h b/esp8266/ets_alt_task.h
index 4b5ba26..dba0c5f 100644
--- a/esp8266/ets_alt_task.h
+++ b/esp8266/ets_alt_task.h
@@ -1,2 +1,4 @@
 extern int ets_loop_iter_disable;
+extern uint32_t system_time_high_word;
+
 bool ets_loop_iter(void);