Fix fatal error, because of new files added in WP#23466

See https://core.trac.wordpress.org/changeset/23466


````
$ wp
PHP Fatal error:  Call to undefined function get_post_format_slugs() in /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-includes/theme.php on line 1270
PHP Stack trace:
PHP   1. {main}() /Users/Dominik/Projekte/wp-cli/php/boot-fs.php:0
PHP   2. include() /Users/Dominik/Projekte/wp-cli/php/boot-fs.php:17
PHP   3. require() /Users/Dominik/Projekte/wp-cli/php/wp-cli.php:25
PHP   4. do_action() /Users/Dominik/Projekte/wp-cli/php/wp-settings-cli.php:284
PHP   5. call_user_func_array() /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-includes/plugin.php:406
PHP   6. twentytwelve_setup() /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-includes/plugin.php:406
PHP   7. add_theme_support() /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-content/themes/twentytwelve/functions.php:61

Fatal error: Call to undefined function get_post_format_slugs() in /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-includes/theme.php on line 1270

Call Stack:
    0.0002     639008   1. {main}() /Users/Dominik/Projekte/wp-cli/php/boot-fs.php:0
    0.0004     655296   2. include('/Users/Dominik/Projekte/wp-cli/php/wp-cli.php') /Users/Dominik/Projekte/wp-cli/php/boot-fs.php:17
    0.0076    2005080   3. require('/Users/Dominik/Projekte/wp-cli/php/wp-settings-cli.php') /Users/Dominik/Projekte/wp-cli/php/wp-cli.php:25
    0.1346   29820536   4. do_action() /Users/Dominik/Projekte/wp-cli/php/wp-settings-cli.php:284
    0.1346   29822872   5. call_user_func_array() /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-includes/plugin.php:406
    0.1346   29822928   6. twentytwelve_setup() /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-includes/plugin.php:406
    0.1350   29826056   7. add_theme_support() /Users/Dominik/Sites/WordPress/Trunk/wp_single/wp-content/themes/twentytwelve/functions.php:61
```