cgraph: fix mismatched type signatures between 'agobjfn_t' and 'agraphattr_init'

cgraph: fix mismatched type signatures between 'agobjfn_t' and 'agraphattr_init'

Squashes a compiler warning:

  attr.c: In function ‘init_all_attrs’:
  attr.c:534:37: warning: cast between incompatible function types from ‘void
    (*)(Agraph_t *)’ {aka ‘void (*)(struct Agraph_s *)’} to ‘void (*)(Agraph_t
    *, Agobj_t *, void *)’ {aka ‘void (*)(struct Agraph_s *, struct Agobj_s *,
    void *)’} [-Wcast-function-type]
    534 |     agapply(root, (Agobj_t *) root, (agobjfn_t) agraphattr_init,
        |                                     ^

This code was incorrect. But on the most common architecture in contemporary
use, x86-64, the calling convention for both is fully in registers, so no user
visible misbehavior would have been seen.

Gitlab: #2300