libpqxx: Add dependency on doxygen and xmlto

pkgs/development/libraries/libpqxx/default.nix

···
-{ lib, stdenv, fetchFromGitHub, postgresql, python2, gnused }:
+{ lib, stdenv, fetchFromGitHub, postgresql, doxygen, xmlto, python2, gnused }:
 
 stdenv.mkDerivation rec {
   name = "libpqxx-${version}";
···
   };
 
   nativeBuildInputs = [ gnused python2 ];
-  buildInputs = [ postgresql ];
+  buildInputs = [ postgresql doxygen xmlto ];
 
   preConfigure = ''
     patchShebangs .