fix NO_HASH_SEED build

fix NO_HASH_SEED build

commit b1300a738f added PERL_HASH_FUNC_ONE_AT_A_TIME_HARD algo, which was
the first one to introduce 8 byte seeds, previously all the algos used 4
or 16 byte seeds. No case was added to the CPP tree for 8 byte const
seeds, so add one now. Otherwise the #error at the end of the tree runs
and breaks the build. NO_HASH_SEED define was public API in the past and
could be considered to still be public API, see commit f36626324a.
My use for NO_HASH_SEED is reducing entropy for tracking down memory
corruption.