For users without CONFIG_IGB_PTP=y, we should not be compiling any PTP
code into the driver. Tidy up the wrapping in igb to support this.

Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: NMatthew Vick <matthew.vick@intel.com>
Acked-by: NJacob Keller <jacob.e.keller@intel.com>
Tested-by: NJeff Pieper  <jeffrey.e.pieper@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>