Add EXPORT to multicast.h

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

src/multicast.h

@@ -56,6 +56,6 @@ struct multicast_broadcast
 };
 
 // returns # of bytes sent
-int multicast_init(RIG *rig, char *addr, int port);
-int multicast_send(RIG *rig, const char *msg, int msglen);
+extern HAMLIB_EXPORT (int) multicast_init(RIG *rig, char *addr, int port);
+extern HAMLIB_EXPORT (int) multicast_send(RIG *rig, const char *msg, int msglen);
 #endif //MULTICAST_H