Use maintenance_mode instead of site_offline setting

The bug resides in the usage of the site_offline setting which was migrated to maintenance_mode in Drupal 7 (see update.inc).

function xmlsitemap_engines_can_submit() {
  // Skip if the site is offline since search engines will not be able to
  // access the site's content.
  if (variable_get('site_offline', 0)) {
    return FALSE;
  }

  if (!variable_get('xmlsitemap_engines_engines', array()) && !variable_get('xmlsitemap_engines_custom_urls', '')) {
    return FALSE;
  }

  return TRUE;
}

See similar issue at Redirect module #1363630: Redirect uses site_offline variable which isnt used by Drupal 7 anymore