i40e: remove linux/export.h header from i40e_ptp.c

We don't need the export.h header so we can just go ahead and remove it.

Change-ID: I9057396b141ee449d8299409081358b9270a7c4d
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>