Privacy: Include `wp-admin/includes/file.php` to avoid fatal error.

`list_files()` is defined in `wp-admin/includes/file.php`, which is not included by `wp-cron.php`, so it needs to be included by the caller in order to avoid a fatal PHP error.

This bug was not detected during testing because the file _is_ included when executing jobs via `wp cron event run`.

Props mikejolley, iandunn.
See #43546.
See https://wordpress.slack.com/archives/C9695RJBW/p1525190405000860.


git-svn-id: https://develop.svn.wordpress.org/trunk@43059 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -6281,6 +6281,8 @@ function wp_schedule_delete_old_privacy_export_files() {
  * @since 4.9.6
  */
 function wp_privacy_delete_old_export_files() {
+	require_once( ABSPATH . 'wp-admin/includes/file.php' );
+
 	$upload_dir   = wp_upload_dir();
 	$exports_dir  = trailingslashit( $upload_dir['basedir'] . '/exports' );
 	$export_files = list_files( $exports_dir, 100, array( 'index.html' ) );