iommu/vt-d: Remove extern from function prototypes

The kernel coding style does not require 'extern' in function prototypes
in .h files, so remove them from drivers/iommu/intel/iommu.h as they are
not needed.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20230331045452.500265-1-baolu.lu@linux.intel.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>