delaunay_triangulation: fix mismatch of calling convention in callback

delaunay_triangulation: fix mismatch of calling convention in callback

The compiler said about this code:

  delaunay.c:286:34: warning: cast between incompatible function types from
    ‘void (*)(GtsSegment *, v_data *)’ {aka ‘void (*)(struct _GtsSegment *,
    struct <anonymous> *)’} to ‘gint (*)(void *, void *)’ {aka
    ‘int (*)(void *, void *)’} [-Wcast-function-type]
       gts_surface_foreach_edge (s, (GtsFunc) add_edge, delaunay);
                                    ^

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