restool: also depend on which

The ls-main script uses `which`. It also has /bin/sh as its shebang,
which means bash runs in Bourne-compatible mode and `which` is not a
builtin.

When the ls-* scripts are invoked from a user shell you don't notice
the problem, because /run/current-system/sw/bin/which is on $PATH, but
when they are invoked in a systemd service it would fail:

.../bin/ls-addni: line 1054: which: command not found

pkgs/os-specific/linux/restool/default.nix

···
-{ stdenv, lib, fetchgit, bash, coreutils, dtc, file, gawk, gnugrep, gnused }:
+{ stdenv, lib, fetchgit, bash, coreutils, dtc, file, gawk, gnugrep, gnused, which }:
 
 stdenv.mkDerivation rec {
   pname = "restool";
···
   };
 
   nativeBuildInputs = [ file ];
-  buildInputs = [ bash coreutils dtc gawk gnugrep gnused ];
+  buildInputs = [ bash coreutils dtc gawk gnugrep gnused which ];
 
   makeFlags = [
     "prefix=$(out)"