package/tvheadend: add dbus as optional dependency

Dbus is an optional dependency for tvheadend:
https://github.com/tvheadend/tvheadend/commit/9c7b6047d2e3040e14cdba89f46d8acb2a36a115



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>