Fix multicast.c build on mingw

https://github.com/Hamlib/Hamlib/issues/695

src/multicast.c

@@ -3,9 +3,14 @@
 #include <string.h>
 #include <errno.h>
 #include <unistd.h>
+#ifdef _WIN32
+#include <winsock2.h>
+#include <ws2tcpip.h>
+#else
 #include <sys/socket.h>
-//#include <netinet/in.h>
+#include <netinet/in.h>
 #include <arpa/inet.h>
+#endif
 #include "hamlib/rig.h"
 #include "misc.h"
 #include "multicast.h"