x86: Don't put iommu_shutdown_noop() in init section

It causes kernel panic on shutdown or reboot.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
LKML-Reference: <4B00BC8E.50801@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>