selftests/net: add missing gitignores (ipv6_flowlabel)

ipv6_flowlabel and ipv6_flowlabel_mgr are missing from
gitignore.  Quentin points out that the original
commit 3fb321fd ("selftests/net: ipv6 flowlabel")
did add ignore entries, they are just missing the "ipv6_"
prefix.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>