addFace: fix mismatch of calling convention in callback

addFace: fix mismatch of calling convention in callback

The compiler said about this code:

  delaunay.c: In function ‘addFace’:
  delaunay.c:428:48: warning: cast between incompatible function types from
    ‘void (*)(GFace *, ninfo *)’ {aka ‘void (*)(struct <anonymous> *, struct
    <anonymous> *)’} to ‘gint (*)(void *, void *)’ {aka ‘int (*)(void *,
    void *)’} [-Wcast-function-type]
       gts_face_foreach_neighbor ((GtsFace*)f, 0, (GtsFunc) addNeighbor, &ni);
                                                  ^

Similar to the prior commit, this was relying on a coincident return value of 0
from the callback function.