[llvm] [bazel] Add explicit dep on protobuf (PR #168928)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

> should be totally safe, I can submit a PR to do that but i want to land this one first because it will also touch the MODULE.bazel.lock. it would be reasonably safe to gitignore this file if we wanted to, the value it provides being checked in for this repo is probably minimal and it will be a bit annoying as it drifts if folks don't realize they should check in their diffs

Allegedly, the lockfile format is designed to minimize merge conflicts. But if it becomes a regular problem, sure.

https://github.com/llvm/llvm-project/pull/168928