iommu/amd: Fix section warning for prealloc_protection_domains

Fix the following section warning in drivers/iommu/amd_iommu.c :

WARNING: vmlinux.o(.text+0x526e77): Section mismatch in reference from the function prealloc_protection_domains() to the function .init.text:alloc_passthrough_domain()
The function prealloc_protection_domains() references
the function __init alloc_passthrough_domain().
This is often because prealloc_protection_domains lacks a __init
annotation or the annotation of alloc_passthrough_domain is wrong.

Cc: stable@vger.kernel.org
Signed-off-by: Steffen Persvold <sp@numascale.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>