package/samba4: add optional dependency to dbus

samba4 picks up dbus as dependency if it was built before:

Checking for dbus                : yes
Checking for header dbus/dbus.h  : yes
Checking for library dbus-1      : yes

There is no configure option to control dbus support so we just make
sure dbus is built before samba4 to have reproducible builds.

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