file.c: export rb_stat_new

* file.c (stat_new_0): constify.
* file.c (rb_stat_new): constify and export.  based on a patch by
  Hanmac (Hans Mackowiak) at [ruby-core:53225].  [Feature #8050]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43107 b2dd03c8-39d4-4d8f-98ff-823fe69b080e