selftests/net: add nettest to .gitignore

nettest is missing from gitignore.

Fixes: acda655f ("selftests: Add nettest")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/.gitignore

@@ -21,3 +21,4 @@ ipv6_flowlabel
 ipv6_flowlabel_mgr
 so_txtime
 tcp_fastopen_backup_key
+nettest