fix: Intel compiler's warning for XXH_ASSERT()

[t-mat]

Intel compiler expects unsigned char as an argument of __builtin_assume().
Without this patch, it reports the following warnings to all XXH_ASSERT()s.

In file included from xxhash.c(43):
xxhash.h(6737): warning #2259: non-pointer conversion from "int" to "unsigned char" may lose significant bits
      XXH_ASSERT(secretBuffer != NULL);
      ^

[Cyan4973]

This makes me think :
do we have an icc test in CI ?

[t-mat]

do we have an icc test in CI ?

Yeah. Sometimes we have icc related issue, it should be in the CI.

Since there's official blog article, it seems it can be done relatively easily.

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