util: Install python six module in gcn dockerfile

six is used in develop, but wasn't used in the GCN staging branch.

Change-Id: Ic1ca42df871d1e683c288282497267d00421609f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32235
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

util/dockerfiles/gcn-gpu/Dockerfile

@@ -23,6 +23,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     python-dev \
     python \
     python-yaml \
+    python-six \
     wget \
     libpci3 \
     libelf1 \