packaging: enable xdg-shell for minimization

Activate xdg-shell, so the "minimize" button works under Tizen

We enable the build-time option "xdg-shell" for Tizen. Tested OK
with the latest Tizen Common image, it should work reliably with
upcoming snapshots & releases as we are taking care of
protocol versions compatibility.

Change-Id: I1474cf48a42bd693dc68e0296a8e4e3d815042ed
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>