Mention libxml2-utils in our asciidoc dependencies

Zax from #tor noticed that while we list docbook-xsl and docbook-xml
in our helpful error text when making the docs fails, we forgot to
also list libxml2-utils. Let's add that.

doc/asciidoc-helper.sh

@@ -49,8 +49,8 @@ elif [ "$1" = "man" ]; then
       echo "==================================";
       echo;
       echo "a2x is installed, but some required docbook support files are";
-      echo "missing. Please install docbook-xsl and docbook-xml (Debian)";
-      echo "or similar.";
+      echo "missing. Please install docbook-xsl, docbook-xml, and";
+      echo "libxml2-utils (Debian) or similar.";
       echo;
       echo "==================================";
       exit 1;