qt54.qtbase: don't propagate postgresql dependency

It is not necessary to propagate the postgresql buildInput if Qt is
configured with PostgreSQL enabled.

pkgs/development/libraries/qt-5/5.4/qtbase/default.nix

@@ -188,14 +188,14 @@ stdenv.mkDerivation {
   # Qt doesn't directly need GLU (just GL), but many apps use, it's small and
   # doesn't remain a runtime-dep if not used
   ++ lib.optionals mesaSupported [ mesa mesa_glu ]
-  ++ lib.optional (postgresql != null) postgresql
   ++ lib.optionals gtkStyle [gnome_vfs libgnomeui gtk GConf];
 
   buildInputs =
     [ bison flex gperf ruby ]
     ++ lib.optional developerBuild gdb
     ++ lib.optional (cups != null) cups
-    ++ lib.optional (mysql != null) mysql.lib;
+    ++ lib.optional (mysql != null) mysql.lib
+    ++ lib.optional (postgresql != null) postgresql;
 
   nativeBuildInputs = [ python perl pkgconfig ];