Include config.hh to get needed macros

[scpeters]

🦟 Bug fix

Part of gazebosim/gz-cmake#412.

Summary

This includes gz/msgs/config.hh wherever the GZ_MSGS_VERSION_NAMESPACE macro is used in order to provide the macro definition.

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)
• [X] codecheck passed (See contributing)
• [X] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (fbc0480) 97.41% compared to head (ea4d420) 97.41%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #426 +/- ## ======================================= Coverage 97.41% 97.41% ======================================= Files 27 27 Lines 1201 1201 ======================================= Hits 1170 1170 Misses 31 31 ```

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