Screen: Add zero-init for _gst_private padding in GstAppSink struct.

To fix "missing initializer for GstAppSink _gst_private" compiler warning.
Lets see if this still compiles on Ubuntu 20.04/22.04 or not, to maybe
get rid of the annoying warning. -> Test pending.

PsychSourceGL/Source/Common/Screen/PsychMovieSupportGStreamer.c

@@ -1287,6 +1287,7 @@ static GstAppSinkCallbacks videosinkCallbacks = {
     PsychNewPrerollCallback,
     PsychNewBufferCallback,
     0,
+    0,
     0
 };
 

PsychSourceGL/Source/Common/Screen/PsychVideoCaptureSupportGStreamer.c

@@ -683,6 +683,7 @@ static GstAppSinkCallbacks videosinkCallbacks = {
     PsychNewPrerollCallback,
     PsychNewBufferCallback,
     0,
+    0,
     0
 };