Add option to disable the volk target

[TheMostDiligent]

At the moment, the volk target is always created by CMake. However, when VOLK_PULL_IN_VULKAN is OFF, the target is not complete and will not build unless path to Vulkan headers is explicitly added. When VOLK_PULL_IN_VULKAN is OFF, the application most likely only needs the volk_headers target and has to explicitly remove volk from build.
It would be very helpful if there was an option to only define the volk_headers target, e.g. VOLK_HEADERS_ONLY

[zeux]

Something like VOLK_HEADERS_ONLY would make sense as a CMake option. Feel free to contribute a PR!