fix build and installation of queuedcustomtype example

it was not built at all (and didn't build with qt in a namespace), and
consequently was not installed as well.

Change-Id: I24d8ac4dd5d70927c262ad6336e5ee32a0fd003a
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>