project.config: mark Google CLA as accepted for mdb/copybara-git-writers

A part of setting up Copybara-as-a-Service.

Change-Id: Ib990745b5b0a608570ca5f08fd6677107f361d6a
Reviewed-on: https://go-review.googlesource.com/c/All-Projects/+/637817
Reviewed-by: David Chase <drchase@google.com>