aggregator: use GST_STIME_FORMAT for GstClockTimeDiff
aggregator: use GST_STIME_FORMAT for GstClockTimeDiff

No need to manually handle negative value of deadline, GST_STIME_FORMAT does
exactly this.

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer