selftests: reuseaddr_conflict: add missing new line at the end of the output

selftests: reuseaddr_conflict: add missing new line at the end of the output

The netdev CI runs in a VM and captures serial, so stdout and
stderr get combined. Because there's a missing new line in
stderr the test ends up corrupting KTAP:

  # Successok 1 selftests: net: reuseaddr_conflict

which should have been:

  # Success
  ok 1 selftests: net: reuseaddr_conflict

Fixes: 422d8dc6fd3a ("selftest: add a reuseaddr test")
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240329160559.249476-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>