net: selftests: Add stress_reuseport_listen to .gitignore

Add newly added stress_reuseport_listen object to .gitignore file.

Fixes: ec8cb4f6 ("net: selftests: Stress reuseport listen")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>