Add a Doxygen filter for .h files (backport #523)

[mergify]

🦟 Bug fix

Summary

Doxygen can not parse the GZ_DEPRECATED() macro in a .h file because the existing filter pattern was only applied to *.hh files, not *.h files.

Here's an example without this patch:

And with this patch:

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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This is an automatic backport of pull request #523 done by Mergify.

[mergify]

Cherry-pick of 8e87733 has failed:

On branch mergify/bp/ign-cmake2/pr-523
Your branch is up to date with 'origin/ign-cmake2'.

You are currently cherry-picking commit 8e87733.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   doc/doxygen/api.in

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally