ruby/io.h: get rid of conflict on AIX

* include/ruby/io.h: undef POSIX compliants names on AIX, which are no
  longer needed.  patch suggested by edelsohn (David Edelsohn) in
  [ruby-core:53815].  [Bug #8174]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39993 b2dd03c8-39d4-4d8f-98ff-823fe69b080e