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

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

> I picked this because it is the current version being used by bazel 8.0.0

Any reason not to bump it to the latest stable version (8.4.2)?

I have no problem w/ using this version though. We can bump this and the bazel version itself separately.

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