ipv6: add missing tx timestamping on IPPROTO_RAW

Raw sockets support tx timestamping, but one case is missing.

IPPROTO_RAW takes a separate packet construction path. raw_send_hdrinc
has an explicit call to sock_tx_timestamp, but rawv6_send_hdrinc does
not. Add it.

Fixes: 11878b40 ("net-timestamp: SOCK_RAW and PING timestamping")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>