datetime: Explicitly list systemd-timesyncd as a dependency

Closes: #2162.

Something changed in Debian packaging and systemd-timesyncd was not
automatically being installed.

Tests:

- Run functional tests for datatime app.

- Run ./run --list-dependencies and note that systemd-timesyncd is listed.

- packages.debian.org shows that systemd-timesyncd package is available in
Bullseye, Bookworm and sid.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/datetime/__init__.py

@@ -11,6 +11,7 @@ from plinth import app as app_module
 from plinth import menu
 from plinth.daemon import Daemon
 from plinth.modules.backups.components import BackupRestore
+from plinth.package import Packages
 
 from . import manifest
 
@@ -73,6 +74,9 @@ class DateTimeApp(app_module.App):
                               'datetime:index', parent_url_name='system')
         self.add(menu_item)
 
+        packages = Packages('packages-datetime', ['systemd-timesyncd'])
+        self.add(packages)
+
         if self._is_time_managed():
             daemon = Daemon('daemon-datetime', 'systemd-timesyncd')
             self.add(daemon)