Fix warning for unused specialization hash_combiner<8> on 32-bit systems

Make type of std::site_t an additional template parameter to avoid the
warning. While this is conceptually redunant its OK since we only use
it in a helper struct.
(cherry picked from commit 709d82d29d5fb3588c494f9c67ad8c2fa1da9205)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>