dasher: enableParallelBuilding = true

I'm a bit impatient :-) and this seems to build without issues,
even on an idle 32-threaded machine.

pkgs/applications/accessibility/dasher/default.nix

···
     dbus
   ] ++ lib.optional speechSupport speechd;
 
+  enableParallelBuilding = true;
+
   meta = {
     homepage = "https://www.inference.org.uk/dasher/";
     description = "Information-efficient text-entry interface, driven by natural continuous pointing gestures";