rtspclientsink: mark cached caps as maybe-leaked to make leaks tracer happy

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-rtsp-server/-/merge_requests/190>

gst/rtsp-sink/gstrtspclientsink.c

@@ -1090,6 +1090,8 @@ gst_rtsp_client_sink_get_all_payloaders_caps (void)
         goto out;
     }
 
+    GST_MINI_OBJECT_FLAG_SET (caps, GST_MINI_OBJECT_FLAG_MAY_BE_LEAKED);
+
   out:
     g_once_init_leave (&ret, caps);
   }