GStreamer 1.18 changed the serialization of enums.
This patch updates gsttr-stats.py to handle the new format.
In absence of that, the script was failing like this:
```
Traceback (most recent call last):
File "/home/ntrrgc/Apps/gstreamer/./subprojects/gst-devtools/tracer/gsttr-stats.py", line 224, in <module>
runner.run()
File "/home/ntrrgc/Apps/gstreamer/subprojects/gst-devtools/tracer/tracer/analysis_runner.py", line 42, in run
self.handle_tracer_entry(event)
File "/home/ntrrgc/Apps/gstreamer/subprojects/gst-devtools/tracer/tracer/analysis_runner.py", line 27,
in handle_tracer_entry
analyzer.handle_tracer_entry(event)
File "/home/ntrrgc/Apps/gstreamer/./subprojects/gst-devtools/tracer/gsttr-stats.py", line 114, in handle_tracer_entry
key = (_SCOPE_RELATED_TO[sv.values['related-to']] + ":" + str(s.values[sk]))
KeyError: 'thread'
```
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4155>
