ixgbe: Add SR-IOV specific modules to driver Makefile

Add the mailbox and SR-IOV feature modules to the ixgbe driver Makefile.

Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>