mkSurface: fix mismatch of calling convention in 'addFace'

mkSurface: fix mismatch of calling convention in 'addFace'

The compiler said about this code:

  delaunay.c:484:34: warning: cast between incompatible function types from
    ‘void (*)(GFace *, fstate *)’ {aka ‘void (*)(struct <anonymous> *, struct
    <anonymous> *)’} to ‘gint (*)(void *, void *)’ {aka ‘int (*)(void *,
    void *)’} [-Wcast-function-type]
       gts_surface_foreach_face (s, (GtsFunc) addFace, &statf);
                                    ^

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