Add Vulkan SDK eg. to build Qt Vulkan support

[boberfly]

Adding to Qt won't interfere with the host machine if Vulkan is not available (it dynamically loads the symbols at runtime), but it needs the SDK present to build the code needed to create a Vulkan context.

This may also open up future uses of Vulkan eg. for USD hdStorm or OpenSubdiv once they support Vulkan.