docbook_xml_dtd: add version 4.4

We already have 4.3 and 4.5, but for the latest "moreutils" package
update we need version 4.4.

pkgs/data/sgml+xml/schemas/xml-dtd/docbook/4.4.nix

@@ -0,0 +1,13 @@
+{stdenv, fetchurl, unzip}:
+
+import ./generic.nix {
+  inherit stdenv fetchurl unzip;
+  name = "docbook-xml-4.4";
+  src = fetchurl {
+    url = http://www.docbook.org/xml/4.4/docbook-xml-4.4.zip;
+    sha256 = "141h4zsyc71sfi2zzd89v4bb4qqq9ca1ri9ix2als9f4i3mmkw82";
+  };
+  meta = {
+    branch = "4.4";
+  };
+}