ixgbevf: add support for software timestamps

Add a call to skb_tx_timestamp in the ixgbevf_tx_map function. This
enables software timestamping for packets sent over this device driver.
The call is placed just prior to when we notify hardware of the new
packet, in order to software timestamp as close as possible to when the
hardware will transmit.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>