Define GZ_PHYSICS_VERSION_NAMESPACE in config.hh

[scpeters]

🦟 Bug fix

Part of https://github.com/gazebosim/gz-cmake/issues/412

Summary

Ensure that the GZ_PHYSICS_VERSION_NAMESPACE macro is defined in config.hh.in.

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 (c7035e5) 77.91% compared to head (d4a2d1c) 77.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #597 +/- ## ======================================= Coverage 77.91% 77.91% ======================================= Files 140 140 Lines 7766 7766 ======================================= Hits 6051 6051 Misses 1715 1715 ```

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