layers: Renamed extensions struct for devices in CV
layers: Renamed extensions struct for devices in CV

Struct contains instance and device extensions, changed name to cover
both.

Change-Id: I34d315b02b426673acb4fc069d8a6a4112f12178

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