MDEV-19643 : Fix semisync on Windows

Use correct preprocessor definition.
Do not compare socket value with FD_SETSIZE

Author: vaintroub

sql/semisync_master_ack_receiver.h

@@ -211,15 +211,15 @@ class Select_socket_listener
     {
       my_socket socket_id= slave->sock_fd();
       m_max_fd= (socket_id > m_max_fd ? socket_id : m_max_fd);
-#ifndef WINDOWS
+#ifndef _WIN32
       if (socket_id > FD_SETSIZE)
       {
         sql_print_error("Semisync slave socket fd is %u. "
                         "select() cannot handle if the socket fd is "
                         "greater than %u (FD_SETSIZE).", socket_id, FD_SETSIZE);
         return 0;
       }
-#endif //WINDOWS
+#endif //_WIN32
       FD_SET(socket_id, &m_init_fds);
       fds_index++;
     }