igb: Remove unnecessary include of <linux/pci-aspm.h>

The igb driver doesn't need anything provided by pci-aspm.h, so remove
the unnecessary include of it.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Sinan Kaya <okaya@kernel.org>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>