Add XML declaration to XML templates.

Without this, at least one search engine spider was generating a
validation warning.

components/templates/src/builtins/rss.xml

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <rss xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">
     <channel>
         <title>{{ config.title }}</title>

components/templates/src/builtins/sitemap.xml

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <urlset xmlns="https://www.sitemaps.org/schemas/sitemap/0.9/sitemap.xsd">
     {% for sitemap_entry in entries %}
         <url>

components/templates/src/builtins/split_sitemap_index.xml

@@ -1,3 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
 <sitemapindex xmlns="https://www.sitemaps.org/schemas/sitemap/0.9/siteindex.xsd">
     {% for sitemap in sitemaps %}
         <sitemap>