Use the SPIRV-Headers files from the subtree copy in third_party,
instead of the ones in include/spirv.

Bug b/123642959

Change-Id: I79f77d2f659d5623583b2de40536dec0c79e2f7f
Reviewed-on: https://swiftshader-review.googlesource.com/c/24348Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>