selftests/net: add proc_net_pktgen to .gitignore

Ensure git doesn't pick up this new target.

Fixes: 03544faa ("selftest: net: add proc_net_pktgen")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250307031356.368350-1-willemdebruijn.kernel@gmail.com


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