ruby.h: export ruby_safe_level_4_warning

* include/ruby/ruby.h (ruby_safe_level_4_warning): needed by extension
  libraries which check safe level 4.  [ruby-dev:47517] [Bug #8652]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43103 b2dd03c8-39d4-4d8f-98ff-823fe69b080e