vm.c: constify

* vm.c (rb_vm_get_binding_creatable_next_cfp): constify arguments.
  (rb_vm_get_ruby_level_next_cfp): ditto.
  (vm_get_ruby_level_caller_cfp): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48875 b2dd03c8-39d4-4d8f-98ff-823fe69b080e