AMD IOMMU: prealloc_protection_domains should be static

Impact: cleanup, reduce kernel size a bit, avoid sparse warning

Fixes sparse warning:
arch/x86/kernel/amd_iommu.c:1299:6: warning: symbol 'prealloc_protection_domains' was not declared. Should it be static?

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>