misc: Re-add -Werror for the gem5 20.2 development

This reverts commit 2a4357bfd0c688a19cfd6b1c600bb2d2d6fa6151,
https://gem5-review.googlesource.com/c/public/gem5/+/35455

Change-Id: I10de506658bcad542abbb6d2b4f0c416d55bc121
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35495
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed