[qtbase] dbuscommon.pri: Fix source file dependency

Make already the creation of the generated cpp file depend on the
creation of the generated h file, which transitively ensures that
the cpp won't be compiled before the h is there.

Change-Id: Idd3aec8d72dac341e835a3d1ef8f2986dbc1ab9f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>