demos: Make DEMOS_WSI_SELECTION a variable, not an option
demos: Make DEMOS_WSI_SELECTION a variable, not an option

Change-Id: I8e10ce8df3d569c13f0adf6c09e5bd431279bc2d

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