Doxygen: exclude C++ __attribute__

[j-rivero]

🦟 Bug fix

See https://github.com/gazebosim/gz-gui/pull/601#issuecomment-1832371462.

Summary

Exclude attribute keyword from C++ parsing since it is not part of the standard: https://github.com/doxygen/doxygen/issues/9142

Checklist

• [x] 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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