catclock: Add darwin support

pkgs/applications/misc/catclock/default.nix

···
     homepage = http://codefromabove.com/2014/05/catclock/;
     license = with licenses; mit;
     maintainers = with maintainers; [ ramkromberg ];
-    platforms = with platforms; linux;
+    platforms = with platforms; linux ++ darwin;
   };
 }