Posts, Post Types: Mark the wp_global_styles post type as _builtin.

All native or “built-in” post types should be marked as such.

Follow up to [52041].

See #54336.

git-svn-id: https://develop.svn.wordpress.org/trunk@52108 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -452,6 +452,7 @@ function create_initial_post_types() {
 			'label'        => __( 'Global Styles' ),
 			'description'  => 'CPT to store user design tokens',
 			'public'       => false,
+			'_builtin'     => true, /* internal use only. don't use this when registering your own post type. */
 			'show_ui'      => false,
 			'show_in_rest' => false,
 			'rewrite'      => false,