visionOS: Mark CompositorLayer as final

The destructor is already marked final, which clang warns about for a
class that is not itself final (-Wfinal-dtor-non-final-class). Nothing
derives from the class, so mark the class final instead.

Pick-to: 6.12
Change-Id: I2c8adcba15b8b0654b1d0ef1efa72100de8ed97a
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>