fix: fix fallback for lgamma

[AayushSabharwal]

The interpolation syntax was incorrect, and evaluated to

if lgamma !== :lgamma
    lgamma(x) = Base.lgamma(x)
end

Which threw an UndefVarError.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.66%. Comparing base (291ca27) to head (d34b1d8).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #90   +/-   ##
=======================================
  Coverage   92.66%   92.66%           
=======================================
  Files           1        1           
  Lines         150      150           
=======================================
  Hits          139      139           
  Misses         11       11           

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