Add dependency on flatpak

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.com>
Differential Revision: https://phabricator.apertis.org/D5343

debian/control

@@ -16,6 +16,7 @@ Depends:
  ${shlibs:Depends},
  ${misc:Depends},
  ${pythons3:Depends},
+ flatpak,
  python3-argcomplete,
  python3-xdg,
  python3-paramiko,