This way we can reuse the header from other places like -
src/intel/vulkan and src/gallium. Only the former is hooked up atm.

Make sure .gitignore is updated, as well as all the users (the mesa
code does not need any changes).

Also ensure that the file is always created by adding it to the
BUILT_SOURCES target.

Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Kristian Høgsberg Kristensen <krh@bitplanet.net>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>