ruby/io.h: fix rb_io_buffer_t

* include/ruby/io.h (rb_io_buffer_t): fix usage of PACKED_STRUCT().
  it must surround the whole declaration on VC.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46892 b2dd03c8-39d4-4d8f-98ff-823fe69b080e