Added MDNS and driver support for multicast

src/net_builtin.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#if MG_ENABLE_TCPIP
+
 #include "arch.h"
 #include "net.h"
 #include "queue.h"
@@ -99,3 +101,5 @@ struct mg_tcpip_spi {
   void (*end)(void *);              // SPI end: slave select high
   uint8_t (*txn)(void *, uint8_t);  // SPI transaction: write 1 byte, read reply
 };
+
+#endif