enumerator.c: use rb_enumerator_size_func

* enumerator.c (rb_enumeratorize_with_size): use strict definition
  rb_enumerator_size_func.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41661 b2dd03c8-39d4-4d8f-98ff-823fe69b080e