make ONVIF a friend so it can access monitor properties

Author: connortechnology

src/zm_monitor.h

@@ -57,6 +57,7 @@ class MonitorLinkExpression;
 class Monitor : public std::enable_shared_from_this<Monitor> {
   friend class MonitorStream;
   friend class MonitorLinkExpression;
+  friend class ONVIF;
 
  public:
   typedef enum {