Fix add_constraint with nonzero constant in function

[blegat]

See https://github.com/matbesancon/MathOptSetDistances.jl/pull/71#issuecomment-1715095998

[codecov[bot]]

Codecov Report

Merging #72 (b91a912) into master (ed4517d) will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   87.35%   87.37%   +0.01%     
==========================================
  Files           6        7       +1     
  Lines         704      705       +1     
==========================================
+ Hits          615      616       +1     
  Misses         89       89              

see 1 file with indirect coverage changes

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