Make sure ip_mreq is not exported by winsock2.h.
Add ip_mreq, ip_mreq_source and ip_msfilter to ws2tcpip.h.

diff --git a/include/winsock.h b/include/winsock.h
index 66c33d2..02a6cc5 100644
--- a/include/winsock.h
+++ b/include/winsock.h
@@ -596,11 +596,13 @@
  * Multicast group information
  */
 
+#if !defined(__WINE_WINSOCK2__)
 struct WS(ip_mreq)
 {
     struct WS(in_addr) imr_multiaddr;
     struct WS(in_addr) imr_interface;
 };
+#endif
 
 /*
  * WSAStartup