Include xxhash.h from a relative location Previously it was always included from src/common/third_party/xxhash/ This change makes it easier to use an externally provided copy of xxhash. Bug: b/260093525 Change-Id: I27c375c58ac3776a15d5d68483c235ece235b059 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4066278 Commit-Queue: Nicolas Capens <nicolascapens@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>