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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/reuseaddr_conflict.c

···
 	fd1 = open_port(0, 1);
 	if (fd1 >= 0)
 		error(1, 0, "Was allowed to create an ipv4 reuseport on an already bound non-reuseport socket with no ipv6");
-	fprintf(stderr, "Success");
+	fprintf(stderr, "Success\n");
 	return 0;
 }