Catch gtest headers in fix_cc_deps

[jonmeow]

Need to autofix #1218

[pk19604014]

Another alternative is to not require Carbon source to be compilable in strict headers mode. That would save some maintenance costs associated with fix_cc_deps.py script, but I'm not sure whether there are obvious disadvantages?

[jonmeow]

The advantage of enforcing deps is that when other people pull in our repo using Bazel, they're less likely to accidentally get access to hidden headers.

[github-actions[bot]]

We triage inactive PRs and issues in order to make it easier to find active work. If this issue should remain active or becomes active again, please comment or remove the inactive label. The long term label can also be added for issues which are expected to take time. This issue is labeled inactive because the last activity was over 90 days ago.