apt-dater: pull missing xxd dependency

`xxd` was recently moved out of `vim` to `xxd` package. As a result
`apt-dater` started failing the build as
https://hydra.nixos.org/build/247714641:

    xxd -i apt-dater.xml > apt-dater.xml.inc
    bash: line 1: xxd: command not found

The change pulls in missing `xxd` dependency.

pkgs/tools/package-management/apt-dater/default.nix

@@ -1,6 +1,6 @@
 { lib, stdenv, fetchFromGitHub
 , autoreconfHook, pkg-config, gettext
-, vim, glib, libxml2, ncurses, popt, screen
+, xxd, glib, libxml2, ncurses, popt, screen
 }:
 
 stdenv.mkDerivation rec {
@@ -15,11 +15,11 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = [
-    pkg-config autoreconfHook gettext
+    pkg-config autoreconfHook gettext xxd
   ];
 
   buildInputs = [
-    libxml2 ncurses vim glib popt screen
+    libxml2 ncurses glib popt screen
   ];
 
   configureFlags = [ "--disable-history" ];