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

• 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)
• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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