selftests: forwarding: Add some missing configuration symbols

For the forwarding selftests to work, we need network namespaces when
using veth/vrf otherwise ping/ping6 commands like these:

ip vrf exec vveth0 /bin/ping 192.0.2.2 -c 10 -i 0.1 -w 5

will fail because network namespaces may not be enabled.

Signed-off-by: Florian Fainelli <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/forwarding/config

···
 CONFIG_NET_SCH_INGRESS=m
 CONFIG_NET_ACT_GACT=m
 CONFIG_VETH=m
+CONFIG_NAMESPACES=y
+CONFIG_NET_NS=y