hash: fix build with gcc 4.4 and 4.5

hash: fix build with gcc 4.4 and 4.5

gcc 4.4 and 4.5 throws following error:
rte_cuckoo_hash.c:145: error: flexible array member in otherwise empty struct.

This is due to empty length in flexible array, which has been changed to use
size 0 in the declaration of the array.

Fixes: 48a399119619 ("hash: replace with cuckoo hash implementation")

Reported-by: Olga Shern <olgas@mellanox.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>