headers: add NET_IFINDEX_UNSPECIFIED in ifdef.h

Signed-off-by: LIU Hao <lh_mouse@126.com>

mingw-w64-headers/include/ifdef.h

diff --git a/mingw-w64-headers/include/ifdef.h b/mingw-w64-headers/include/ifdef.h
index 0d78a00..896ef6c 100644
--- a/mingw-w64-headers/include/ifdef.h
+++ b/mingw-w64-headers/include/ifdef.h
@@ -19,6 +19,9 @@
 
 typedef ULONG NET_IFINDEX, *PNET_IFINDEX;
 typedef UINT16 NET_IFTYPE, *PNET_IFTYPE;
+
+#define NET_IFINDEX_UNSPECIFIED (NET_IFINDEX)(0)
+
 typedef NET_IFINDEX IF_INDEX, *PIF_INDEX;
 
 typedef GUID NET_IF_NETWORK_GUID;