updated xxhash to latest dev branch commit

[Cyan4973]

this version supports redirection of XXH_memcpy and XXH_memset.
This capability is used during local zstd adaptation to redirect to zstd_deps.h's ZSTD_memcpy and ZSTD_memset.

[Cyan4973]

The visual studio compilation error in xxhash.h is kinda new.

More worrying, I can't make sense of the error message :

D:\a\zstd\zstd\lib\common\xxhash.h(3182): error C4702: unreachable code [D:\a\zstd\zstd\build\VS2010\fuzzer\fuzzer.vcxproj]

When looking at xxhash.h(3182), it does not feel correct.
This code is obviously reachable and reached, it's very simple code, nothing weird happening there.

One possibility is that the line number 3182 might be completely wrong,
but then it doesn't help identifying what could be the problem.

No other compiler triggers this warning.
I also note that the issue only happens in combination with fuzzer.c, compilation of xxhash is fine within libzstd or any other program.