gc.c (struct heap_page): trivial packing

304 => 296 bytes on x86-64

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48391 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-11-13

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48392 b2dd03c8-39d4-4d8f-98ff-823fe69b080e