iommu/amd: Remove extern from function prototypes

The kernel coding style does not require 'extern' in function prototypes.
Hence remove them from header file.

No functional change intended.

Suggested-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20230609090631.6052-2-vasant.hegde@amd.com


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