iommu: Define dev_iommu_fwspec_get() for !CONFIG_IOMMU_API

There are users outside of the IOMMU code that need to call that
function. Define it for !CONFIG_IOMMU_API too so that compilation does
not break.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200326150841.10083-2-joro@8bytes.org

include/linux/iommu.h

@@ -1073,6 +1073,10 @@ static inline int iommu_sva_unbind_gpasid(struct iommu_domain *domain,
 	return -ENODEV;
 }
 
+static inline struct iommu_fwspec *dev_iommu_fwspec_get(struct device *dev)
+{
+	return NULL;
+}
 #endif /* CONFIG_IOMMU_API */
 
 #ifdef CONFIG_IOMMU_DEBUGFS