liferea: enable parallel builds

Before the change liferea took 41 seconds to build on my machine.
After the change it takes 11 seconds. Almost 4x speedup.

pkgs/applications/networking/newsreaders/liferea/default.nix

@@ -60,6 +60,8 @@ stdenv.mkDerivation rec {
     gst-plugins-bad
   ]);
 
+  enableParallelBuilding = true;
+
   pythonPath = with python3Packages; [
     pygobject3
     pycairo