selftests: net: sort .gitignore file

This is the result of `sort tools/testing/selftests/net/.gitignore`, but
preserving the comment at the top.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
Link: https://lore.kernel.org/r/20220829184748.1535580-1-axelrasmussen@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>