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>

lib/librte_hash/rte_cuckoo_hash.c

@@ -142,7 +142,7 @@ struct rte_hash_key {
 		void *pdata;
 	};
 	/* Variable key size */
-	char key[];
+	char key[0];
 } __attribute__((aligned(KEY_ALIGNMENT)));
 
 /** Bucket structure */