dir.c: try match PLAIN

* dir.c (glob_helper): try match PLAIN as well as ALPHA, which are
  separated by previous commits.  [ruby-core:61552] [Bug #9648]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45969 b2dd03c8-39d4-4d8f-98ff-823fe69b080e