rng-tools: Enable cross-compilation

pkgs/tools/security/rng-tools/default.nix

···
     sha256 = "13h7lc8wl9khhvkr0i3bl5j9bapf8anhqis1lcnwxg1vc2v058b0";
   };
 
+  # For cross-compilation
+  makeFlags = [ "AR:=$(AR)" ];
+
   meta = {
     description = "A random number generator daemon";