avmedia/gstreamer: use GST_SECOND

use GST_SECOND instead of magic value

Change-Id: I992ecbab458f113a75658b1ff69df60068324509
Reviewed-on: https://gerrit.libreoffice.org/10705
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>