backintime: add missing dependency 'packaging'

Fixes this error:

  $ ./result/bin/backintime-qt
  /nix/store/fpr65b2gwpa4q9w3k6z8kjcd9yprmg6s-backintime-qt-1.3.3
  Traceback (most recent call last):
    File "/nix/store/fpr65b2gwpa4q9w3k6z8kjcd9yprmg6s-backintime-qt-1.3.3/share/backintime/qt/app.py", line 35, in <module>
      import qttools
    File "/nix/store/fpr65b2gwpa4q9w3k6z8kjcd9yprmg6s-backintime-qt-1.3.3/share/backintime/qt/qttools.py", line 32, in <module>
      from packaging.version import Version
  ModuleNotFoundError: No module named 'packaging'

Fixes #241966.

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

@@ -1,7 +1,7 @@
 { mkDerivation, backintime-common, python3 }:
 
 let
-  python' = python3.withPackages (ps: with ps; [ pyqt5 backintime-common ]);
+  python' = python3.withPackages (ps: with ps; [ pyqt5 backintime-common packaging ]);
 in
 mkDerivation {
   inherit (backintime-common)