Force impicit fallthrough off in xxhash.
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 <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle