The kci_test_encap_fou() test from kci_test_encap() in rtnetlink.sh
needs the fou module to work. Otherwise it will fail with:

  $ ip netns exec "$testns" ip fou add port 7777 ipproto 47
  RTNETLINK answers: No such file or directory
  Error talking to the kernel

Add the CONFIG_NET_FOU into the config file as well. Which needs at
least to be set as a loadable module.

Fixes: 6227efc1 ("selftests: rtnetlink.sh: add vxlan and fou test cases")
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Link: https://lore.kernel.org/r/20201019030928.9859-1-po-hsu.lin@canonical.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>