Fix function pointer disagreement

[hoodmane]

This fixes the error:

galpy/orbit/orbit_c_ext/integratePlanarOrbit.c:439:35: error: incompatible function pointer types assigning to 'double (*)(double, double, double, double, struct potentialArg *)' from 'double (*)(double, double, double, struct potentialArg *)' [-Wincompatible-function-pointer-types]
      potentialArgs->potentialEval= &ZeroPlanarForce;

https://app.circleci.com/pipelines/github/hoodmane/pyodide/3785/workflows/deda3693-7508-459d-a8e1-2f576f821532/jobs/45989

[codecov]

Codecov Report

Merging #499 (3d71b3d) into main (de7da19) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #499   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files         196      196           
  Lines       27353    27353           
=======================================
  Hits        27307    27307           
  Misses         46       46           

Impacted Files	Coverage Δ
galpy/orbit/orbit_c_ext/integratePlanarOrbit.c	100.00% <100.00%> (ø)

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[jobovy]

Thanks for the fix! I guess this slipped through because regular C compilers don't error on this.