Force the Mac configure to require D-Bus

The D-Bus module should be compiled on Mac. The only reason it isn't
getting right now is because the D-Bus headers must have been forgotten
when the new Mac builders were installed. This is a regression from Qt 4
days, when it was provided.

Task-number; QTBUG-6429
Change-Id: Idb2599ba0ad5a3f6c5e690e2c84760c7b00bfa72
Reviewed-by: Simo Fält <simo.falt@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>