HPCC-8621 Reinstate typedef for hash64_t when compiling on windows

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #3869 from ghalliday/issue8621

HPCC-8621 Reinstate typedef for hash64_t when compiling on windows

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>