Add XHTML namespace option

The 5 year old patch to xmlsitemap.admin.inc and xmlsitemap.module does not apply to the latest xmlsitemap 7.x-2.x-dev and if still applicable needs a reroll.

Checking patch xmlsitemap.admin.inc...
Hunk #1 succeeded at 391 (offset 48 lines).
Checking patch xmlsitemap.module...
error: while searching for:
    'xmlsitemap_frontpage_priority' => 1.0,
    'xmlsitemap_frontpage_changefreq' => XMLSITEMAP_FREQUENCY_DAILY,
    'xmlsitemap_lastmod_format' => XMLSITEMAP_LASTMOD_MEDIUM,
    'xmlsitemap_gz' => FALSE,
    // Removed variables are set to NULL so they can still be deleted.
    'xmlsitemap_regenerate_last' => NULL,

error: patch failed: xmlsitemap.module:289
error: xmlsitemap.module: patch does not apply

Checking patch xmlsitemap.xmlsitemap.inc...
Hunk #1 succeeded at 126 (offset 60 lines).
error: while searching for:

  public function getRootAttributes() {
    $attributes['xmlns'] = 'http://www.sitemaps.org/schemas/sitemap/0.9';
    if (variable_get('xmlsitemap_developer_mode', 0)) {
      $attributes['xmlns:xsi'] = 'http://www.w3.org/2001/XMLSchema-instance';
      $attributes['xsi:schemaLocation'] = 'http://www.sitemaps.org/schemas/sitemap/0.9 http://www.sitemaps.org/schemas/sitemap/0.9/siteindex.xsd';

error: patch failed: xmlsitemap.xmlsitemap.inc:175
error: xmlsitemap.xmlsitemap.inc: patch does not apply

The latest patches from #1670086: using rel="alternate" rather than multiple sitemaps by language context include the scope of this ticket, so let's close this one as Duplicate. They add the required XHTML namespace.