Allow compiling DCB related files if I40E_DCB option
is supported in the kernel configuration.

DCB is disabled by default.
Signed-off-by: NNeerav Parikh <Neerav.Parikh@intel.com>
Signed-off-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: NKavindya Deegala <kavindya.s.deegala@intel.com>
Tested-By: Jack Morgan<jack.morgan@intel.com>
Signed-off-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>