Define GZ_GUI_VERSION_NAMESPACE in config.hh

[scpeters]

🦟 Bug fix

Part of gazebosim/gz-cmake#412.

Summary

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

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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (838f6bc) 70.06% compared to head (1e327c2) 70.09%.
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
+ Coverage   70.06%   70.09%   +0.02%     
==========================================
  Files          39       38       -1     
  Lines        5359     5350       -9     
==========================================
- Hits         3755     3750       -5     
+ Misses       1604     1600       -4     

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