Makefile: Fix race that causes us to miss Protobuf changes

The check-proto target gets executed as part of the verify target by our CI to assert that code generated from our Protobuf definitions matches the committed code and that it passes our linting rules. But the former part to check for changes is racy: check-proto depends on both proto and no-proto-changes in order to first regenerate files and then check for changes. But if make(1) is executed with multiple jobs these can run in parallel as no-proto-changes does not depend on proto. Because executing git-diff(1) is faster than generating the code the end result is that we don't ever notice out-of-date code.

Fix this error by fixing the dependency chain so that no-proto-changes depends on proto.