neatogen: remove duplicate declaration of in_poly in neatosplines.c

neatogen: remove duplicate declaration of in_poly in neatosplines.c

The in_poly function is already declared in lib/pathplan/pathutil.h.

Without this change, an upcoming commit that adds dllimport/dllexport
storage-class attribute declarations in lib/pathplan/pathutil.h would
cause the following warning for the duplicate definition:

C:\Users\magja\graphviz\lib\neatogen\neatosplines.c(27,48): warning C4273: 'in_poly': inconsistent dll linkage [C:\Users\magja\graphviz\build\lib\neatogen\neatogen.vcxproj]

Towards https://gitlab.com/graphviz/graphviz/-/issues/2058.