Fix build with gstreamer 1.20.x

gstreamer-plugins-base 1.20 includes a new member in the GstAppSinkCallbacks struct:

gboolean (*new_event) (GstAppSink *appsink, gpointer user_data);

So it has to be initialized in order to build test-gst.cpp successfully.

(added in gstreamer/gstreamer@0a657d6d )