Enable IP_ROUTING for netperf UDP_STREAM test

Enable IP_ROUTING for netperf UDP_STREAM test

To avoid the following ERROR when netperf test between
2 different subnets:
ERROR netperf: send_omni: send_data failed: Network is unreachable

For detail, please see:
https://serverfault.com/questions/802320/netperf-iptables-\
masquerade-network-unreachable
Or:
https://stackoverflow.com/questions/11981480/error-in-running-\
netperf-udp-stream-over-openvpn

Change-Id: I62b202844861440deaf3bf0f65b41561bd87ae87
Signed-off-by: Trevor Tao <trevor.tao@arm.com>