libaudit: Add stub

pkgs/top-level/all-packages.nix

···
   atop = callPackage ../os-specific/linux/atop { };
 
   audit = callPackage ../os-specific/linux/audit { };
+  libaudit = audit;
 
   b43Firmware_5_1_138 = callPackage ../os-specific/linux/firmware/b43-firmware/5.1.138.nix { };