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