mem-ruby: Replace the deprecated Stats namespace

Using the old namespace generates quite a few warnings during
compilation time.

Change-Id: Ibbb8f54f0bee974443dc5dfcc30e95100b8c24d6
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60289
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
1 file changed