Mark natvis file as PUBLIC in CMakeLists.txt

[k3DW]

Closes #307 , in tandem with boostorg/cmake#89

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (cc9a787) to head (d5871cd).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #317   +/-   ##
========================================
  Coverage    98.23%   98.23%           
========================================
  Files          161      161           
  Lines        22223    22223           
========================================
  Hits         21831    21831           
  Misses         392      392           

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