ixgbe: remove linux/export.h header from ixgbe_ptp.c

We don't need this header file, so we shouldn't be including it.

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>