vulkan/util: add struct vk_pipeline_cache_header

Header is defined at vkGetPipelineCacheData spec, in any vulkan
version, and anv, tu and radv were using the same struct, and v3dv was
about to do the same.

Defining the same struct four times seemed odd, so let's define on a
common place.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6058>