gen/genX_pack: Add genxml to the pack header path

gen/genX_pack: Add genxml to the pack header path

If you have an out-of-tree build, gen8_pack.h and friends will not be in
the same folder as genX_pack.h so this will be a problem.  We fixed
out-of-tree earlier by adding the genxml folder to the includes for the
vulkan driver.  However, this is not a good long-term solution because we
want to use it in ISL as well.