postgresql: enable XML functions

I suspect these functions aren't widely used, but they are enabled in
PostgreSQL on Ubuntu and Arch.

pkgs/servers/sql/postgresql/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, glibc, fetchurl, zlib, readline, libossp_uuid, openssl, makeWrapper }:
+{ lib, stdenv, glibc, fetchurl, zlib, readline, libossp_uuid, openssl, libxml2, makeWrapper }:
 
 let
 
@@ -15,7 +15,7 @@ let
     setOutputFlags = false; # $out retains configureFlags :-/
 
     buildInputs =
-      [ zlib readline openssl makeWrapper ]
+      [ zlib readline openssl libxml2 makeWrapper ]
       ++ lib.optionals (!stdenv.isDarwin) [ libossp_uuid ];
 
     enableParallelBuilding = true;
@@ -24,6 +24,7 @@ let
 
     configureFlags = [
       "--with-openssl"
+      "--with-libxml"
       "--sysconfdir=/etc"
       "--libdir=$(lib)/lib"
     ]