dir.c: set PLAIN for non-magical

* dir.c (glob_make_pattern): set PLAIN for non-magical path to
  skip parts which not need to glob.
  [ruby-core:61552] [Bug #9648]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45968 b2dd03c8-39d4-4d8f-98ff-823fe69b080e