demos: Fix CMake file to allow WSI selection
demos: Fix CMake file to allow WSI selection

Define DEMOS_WSI_SELECTION CMake variable so it can be set
from CMake command line or GUI.

Change-Id: If2531d102fe9b054a1eee4da28431779b173fc10

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers