restore killed_ground_units as it is relied on to track scenery kills

This PR fixes a regression introduced in
https://github.com/dcs-liberation/dcs_liberation/pull/2792 where
refactoring meant that scenery deaths were not tracked correctly.

This PR has been tested by striking a scenery target and confirming that
it appears in state.json and is updated in Liberation. I've also
confirmed that ground units are tracked.

game/debriefing.py

@@ -133,6 +133,7 @@ class StateData:
             + data["kill_events"]
             + data["crash_events"]
             + data["dead_events"]
+            + data["killed_ground_units"]
         )
         for unit in killed_units:  # organize killed units into aircraft vs ground
             if unit_map.flight(unit) is not None: