ruby.h: err rb_check_safe_str explicitly

* include/ruby/ruby.h (rb_check_safe_str, Check_SafeStr): show
  error message to update for obsolete function and macro.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47636 b2dd03c8-39d4-4d8f-98ff-823fe69b080e