package/systemd: do not force dbus if dbus-broker is available

dbus-broker fits the bill as a message bus daemon, so only enable the
original dbus if dbus-broker is not enabled.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Norbert Lange <nolange79@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>