fix(mmgs): add LIBMMGS_EXPORT to MMGS_Get_triangleQuality declaration

[kmarchais]

MMGS_Get_triangleQuality is missing LIBMMGS_EXPORT in src/mmgs/libmmgs.h. Every other API function in the header has it. Without it the symbol is not exported from libMmgs.dll on Windows, so downstream consumers of a prebuilt DLL (e.g. conda-forge mmgsuite) hit an unresolved external symbol.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.30%. Comparing base (ae7d305) to head (90bad59).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #325      +/-   ##
===========================================
- Coverage    50.31%   50.30%   -0.01%     
===========================================
  Files          177      177              
  Lines        47831    47840       +9     
  Branches     10352    10349       -3     
===========================================
+ Hits         24065    24067       +2     
- Misses       16037    16044       +7     
  Partials      7729     7729              

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