igc: Remove forward declaration

Move igc_adapter and igc_ring structures up to avoid
forward declaration
It is not necessary to forward declare these structures
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>