Make wayland-scanner install generated headers

QtWayland uses QtWaylandScanner to generate c++ files which will be
subclassed. This works ok in QtCompositor as it implements the pimple
pattern. However, QtWayland has to expose a library called
QtWaylandClient as a way to create specialized wayland plugins. These
classes don't promise an ABI, and has limited API guarantees, so they
subclass the classes generated by the QtWaylandScanner directly. For
this library to be possible, we have to install the generated files.
Also, it should be possible to use the private API, and that's not
possible without the generated files.

Change-Id: Id55200fe23652587390dabf84ca846e4f39cf70e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>