Define GZ_PHYSICS_VERSION_NAMESPACE in config.hh

[scpeters]

🦟 Bug fix

Part of gazebosim/gz-cmake#412

Summary

Ensure that the GZ_PHYSICS_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 (c7035e5) 77.91% compared to head (d4a2d1c) 77.91%.

Additional details and impacted files

@@           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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