examples/ipv4_multicast: enable multicast promiscuous

examples/ipv4_multicast: enable multicast promiscuous

This example has not been enable for receiving multicast
packet, so it will drop multicast packet. Users must send packet
with ether MAC destination address the same as pf port MAC address,
in order to forward packet successfully, but this is an example
for forwarding ipv4 multicastpacket. So calling function
rte_eth_promiscuous_enable() or rte_eth_allmulticast_enable() can
enable promiscuous mode of all multicast packet. And also, DPDK has
rte API function of rte_eth_dev_set_mc_addr_list() for setting
specific multicast filter table for specific multicast IP address,
but this example do not support this configuration, so it need to
be enable multicast promiscuous mode instead.

Signed-off-by: Wei Zhao <wei.zhao1@intel.com>
Tested-by: Dong Wang <dong1.wang@intel.com>