backintime-common: add missing dependency 'packaging'

Fixes this fatal startup error:

$ nix-build -A backintime-common && ./result/bin/backintime
[...]
ModuleNotFoundError: No module named 'packaging'

Fixes https://github.com/NixOS/nixpkgs/issues/241966.

pkgs/applications/networking/sync/backintime/common.nix

···
 python3, rsync, cron, openssh, sshfs-fuse, encfs }:
 
 let
-  python' = python3.withPackages (ps: with ps; [ dbus-python keyring ]);
+  python' = python3.withPackages (ps: with ps; [ dbus-python keyring packaging ]);
 
   apps = lib.makeBinPath [ openssh python' cron rsync sshfs-fuse encfs ];
 in stdenv.mkDerivation rec {