Only add one body class when multiple post types manage to be present for a post type archive due to pre_get_posts manipulation.

fixes #25341.

Built from https://develop.svn.wordpress.org/trunk@25745


git-svn-id: http://core.svn.wordpress.org/trunk@25658 1a063a9b-81f0-0310-95a4-ce76da25c4cd