supply AI_V4MAPPED

Index: code/network/multi_portfwd.cpp
--- code/network/multi_portfwd.cpp.orig
+++ code/network/multi_portfwd.cpp
@@ -7,6 +7,9 @@
  *
 */
 
+/* not supplied in headers */
+#define AI_V4MAPPED                 0x00000800
+
 #ifdef _WIN32
 #include <winsock2.h>
 #include <ws2tcpip.h>
