iommu/vt-d: mark internal functions as static

Functions alloc_iommu() and parse_ioapics_under_ir()
are only used internally, so mark them as static.

[Joerg: Made detect_intel_iommu() non-static again for IA64]

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>