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>

drivers/iommu/amd_iommu.c

@@ -103,7 +103,7 @@ struct flush_queue {
 	struct flush_queue_entry *entries;
 };
 
-DEFINE_PER_CPU(struct flush_queue, flush_queue);
+static DEFINE_PER_CPU(struct flush_queue, flush_queue);
 
 static atomic_t queue_timer_on;
 static struct timer_list queue_timer;