ixgbe: support software timestamping

Kernel software timestamping requires that the driver calls skb_tx_timestamp
just before passing the skb to the MAC, in order to provide the best software
timestamps. This patch adds this call for that support.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>