news: Add an empty <change> to each <section>

The generated HTML will contain <ul></ul> otherwise, which
triggers an error during 'make check'.

The proper fix would be not to generate the problematic
HTML in the first place but, while I'm working on it, this
workaround will do.

docs/news.xml

@@ -12,10 +12,13 @@
 <libvirt>
   <release version="v3.2.0" date="unreleased">
     <section title="New features">
+      <change/>
     </section>
     <section title="Improvements">
+      <change/>
     </section>
     <section title="Bug fixes">
+      <change/>
     </section>
   </release>
   <release version="v3.1.0" date="2017-03-03">