vm_method.c: change argument of set_visibility

* vm_method.c (set_visibility): use rb_method_flag_t as well as
  set_method_visibility() and rename argument.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41110 b2dd03c8-39d4-4d8f-98ff-823fe69b080e