Fix GN warning about unused variable when angle_enable_commit_id=false BUG=angleproject:2344 Change-Id: I3a0c8384d280755557fd425e80877addd756a9ef Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2091625 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>