xautolock: make linux-only

pkgs/misc/screensavers/xautolock/default.nix

···
     description = "A program that launches a given program when your X session has been idle for a given time.";
     homepage = http://www.ibiblio.org/pub/linux/X11/screensavers;
     maintainers = with maintainers; [ garbas ];
-    platforms = platforms.unix;
+    platforms = platforms.linux;
     license = licenses.gpl2;
   };
 }