base-stats,python: Add missing "group" in _prepare_stats

Change-Id: Idb25185e1d29ac9fd8c0503d55b56e0568a61d1f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42704
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/stats/gem5stats.py

@@ -230,7 +230,7 @@ def _prepare_stats(group: _m5.stats.Group):
     for stat in group.getStats():
         stat.prepare()
 
-    for child in getStatGroups().values():
+    for child in group.getStatGroups().values():
         _prepare_stats(child)