[core] Workaround LwIPmDNS compilation

arduino/libretuya/libraries/mDNS/LwIPmDNS.cpp

@@ -9,7 +9,7 @@ extern "C" {
 #include <lwip/netif.h>
 }
 
-extern u8_t mdns_netif_client_id;
+static u8_t mdns_netif_client_id = 0; // TODO fix this
 
 struct mdns_domain {
 	/* Encoded domain name */
@@ -60,6 +60,8 @@ bool mDNS::begin(const char *hostname) {
 	mdns_resp_init();
 	struct netif *netif = netif_list;
 	while (netif != NULL) {
+		// TODO: detect mdns_netif_client_id by checking netif_get_client_data()
+		// and finding the requested hostname in struct mdns_host
 		if (netif_is_up(netif) && mdns_resp_add_netif(netif, hostname, 255) != ERR_OK) {
 			return false;
 		}