Fix warning of dropping 'const' signifier.

dacp.c

@@ -474,7 +474,7 @@ void set_dacp_server_information(rtsp_conn_info *conn) {
   debug_mutex_unlock(&dacp_server_information_lock, 3);
 }
 
-void dacp_monitor_port_update_callback(char *dacp_id, uint16_t port) {
+void dacp_monitor_port_update_callback(const char *dacp_id, uint16_t port) {
   debug_mutex_lock(&dacp_server_information_lock, 500000, 2);
   debug(3,
         "dacp_monitor_port_update_callback with Remote ID \"%s\", target ID \"%s\" and port "

dacp.h

@@ -31,7 +31,7 @@ void relinquish_dacp_server_information(rtsp_conn_info *conn); // tell the DACP
                                                                // that the player thread is no
                                                                // longer associated with it.
 void dacp_monitor_port_update_callback(
-    char *dacp_id, uint16_t port); // a callback to say the port is no longer in use
+    const char *dacp_id, uint16_t port); // a callback to say the port is no longer in use
 
 int dacp_send_command(const char *command, char **body, ssize_t *bodysize);
 int send_simple_dacp_command(const char *command);