hash: fix header for C++

hash: fix header for C++

When built in a C++ application, the jhash include fails:

rte_jhash.h:123:22: error:
invalid conversion from ‘const void*’ to ‘const uint32_t*’ [-fpermissive]
  const uint32_t *k = key;
                      ^
The cast must be explicit for C++.

Fixes: 8718219a8737 ("hash: add new jhash functions")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>