examples/distributor: reduce Tx queue number to 1

examples/distributor: reduce Tx queue number to 1

Distributor application creates one Tx queue per core. However
the transmit is done only from a single core. Hence creating
one Tx queue is enough.

Fixes: 07db4a975094 ("examples/distributor: new sample app")
Cc: stable@dpdk.org
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>