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