Expose lwIP's DNS API in zts_* API. Fix preprocessor build bug

src/VirtualTap.hpp

@@ -224,11 +224,6 @@ void _lwip_driver_shutdown();
  */
 void _lwip_remove_netif(void *netif);
 
-/**
- * @brief Initialize and start the DNS client
- */
-void _lwip_dns_init();
-
 /**
  * @brief Starts DHCP timers
  */