iommu: Fix compile error in iommu-sysfs.c

Add missing include of <linux/slab.h>.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
diff --git a/drivers/iommu/iommu-sysfs.c b/drivers/iommu/iommu-sysfs.c
index d693923..39b2d91 100644
--- a/drivers/iommu/iommu-sysfs.c
+++ b/drivers/iommu/iommu-sysfs.c
@@ -12,6 +12,7 @@
 #include <linux/device.h>
 #include <linux/iommu.h>
 #include <linux/module.h>
+#include <linux/slab.h>
 
 /*
  * We provide a common class "devices" group which initially has no attributes.