rb_io_buffer_t: fix packing on gcc

* include/ruby/io.h (rb_io_buffer_t): fix packing on gcc
  r46892 caused packing to be a no-op on gcc (4.7.2-5, Debian)
  [Bug #10079][ruby-core:63912]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46894 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-07-22

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46895 b2dd03c8-39d4-4d8f-98ff-823fe69b080e