Disable -Wcast-qual in qtwayland

We have a lot of generated C source code that needs to do casting and C
doesn't have const_cast, so the -Wcast-qual option produces a lot of
noise. Let's make sure it isn't enabled in qmake.conf or qt_common.prf.

Change-Id: Iee8cbc07c4434ce9b560ffff13c9635861a8eca9
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>