stats: Ignore non-Group objects in stat hierarchy

stats: Ignore non-Group objects in stat hierarchy

Some objects, such as SystemC modules, are not a subclass of
Stat::Group. Calling the addStatGroup function on them causes errors.
This changes ignores those objects that are not Stat::Group in the stat
hierarchy.

Signed-off-by: Chun-Chen TK Hsu <chunchenhsu@google.com>
Change-Id: I9b62419417b7af7331461fbfaf15e45a4ee2b35f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20680
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>