iommu: Change api header to static inline

Update the function header of iommu_fwspec_get_id to static inline
so compilation proceeds when CONFIG_IOMMU_API=n.

Change-Id: Ib903e537cf26d1352a7c13c843269ebcd369a128
Signed-off-by: Runmin Wang <runminw@codeaurora.org>
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 3efd210..1f6892c 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -706,7 +706,7 @@ static inline int iommu_fwspec_add_ids(struct device *dev, u32 *ids,
 	return -ENODEV;
 }
 
-int iommu_fwspec_get_id(struct device *dev, u32 *id)
+static inline int iommu_fwspec_get_id(struct device *dev, u32 *id)
 {
 	return -ENODEV;
 }