Force impicit fallthrough off in xxhash. This silences a warning present in Skia. The warning is disabled in ANGLE/Chrome by build's compiler.gni. Bug: skia:7647 Change-Id: Ie6308b7f4395786564aa79db8eb35401f3d98048 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2382414 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>