iommu/amd: Fix non static symbol warning

Fixes the following sparse warning:

drivers/iommu/amd_iommu.c:106:1: warning:
 symbol '__pcpu_scope_flush_queue' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>