test/distributor: fix return type of thread function

New warning with gcc 8.1:
When casting the function pointer passed to
rte_eal_remote_launch() as an lcore_function_t *,
it expects the cast function to return an int, but
instead it was returning void. Fixed to return
an int.

Fixes: c3eabff124 ("distributor: add unit tests")
Cc: stable@dpdk.org

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>

test/test/test_distributor_perf.c

@@ -31,7 +31,7 @@ struct worker_stats worker_stats[RTE_MAX_LCORE];
  * worker thread used for testing the time to do a round-trip of a cache
  * line between two cores and back again
  */
-static void
+static int
 flip_bit(volatile uint64_t *arg)
 {
 	uint64_t old_val = 0;
@@ -41,6 +41,7 @@ flip_bit(volatile uint64_t *arg)
 		old_val = *arg;
 		*arg = 0;
 	}
+	return 0;
 }
 
 /*