mgr::priv -> mgr::ifp, use explicit type

src/net_builtin.h

@@ -3,10 +3,9 @@
 #include "arch.h"
 #include "net.h"
 #include "queue.h"
+#include "str.h"
 
-#if defined(MG_ENABLE_TCPIP) && MG_ENABLE_TCPIP
 struct mg_tcpip_if;  // Mongoose TCP/IP network interface
-#define MG_TCPIP_IFACE(mgr_) ((struct mg_tcpip_if *) (mgr_)->priv)
 
 struct mg_tcpip_driver {
   bool (*init)(struct mg_tcpip_if *);                         // Init driver
@@ -91,5 +90,3 @@ 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