x86: don't include {amd-,}iommu.h from fixmap.h

x86: don't include {amd-,}iommu.h from fixmap.h

The #include was added by 0700c962ac ("Add AMD IOMMU support into
hypervisor") and I then didn't drop it again in d7f913b8de ("AMD IOMMU:
use ioremap()"); similarly for xen/iommu.h in 99321e0e6c ("VT-d: use
ioremap()"). Avoid needlessly re-building unrelated files when only
IOMMU definitions have changed.

Two #include-s of xen/init.h turn out necessary as replacement.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>