cleanup: enable clang-tidy in pubsub headers (#3948)
cleanup: enable clang-tidy in pubsub headers (#3948)

clang-tidy does not scan headers by default, you have to enable a
regular expression for it. As we import projects where the headers do
pass the clang-tidy checks, we should enable the checks for that
particular project.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp