contrib: fixed c++ compatibility of unix port

contrib/ports/unix/port/include/netif/fifo.h

@@ -3,6 +3,10 @@
 
 #include "lwip/sys.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** How many bytes in fifo */
 #define FIFOSIZE 2048
 
@@ -50,5 +54,9 @@ void fifoPut(fifo_t * fifo, int fd);
 */
 void fifoInit(fifo_t * fifo);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif