iommu/iova: Fix compile error with CONFIG_IOMMU_IOVA=m

The #ifdef in iova.h only catches the CONFIG_IOMMU_IOVA=y
case, so that compilation as a module fails with duplicate
function definition errors. Fix it by catching both cases in
the #if.
Signed-off-by: Joerg Roedel <jroedel@suse.de>

include/linux/iova.h

@@ -82,7 +82,7 @@ static inline unsigned long iova_pfn(struct iova_domain *iovad, dma_addr_t iova)
 	return iova >> iova_shift(iovad);
 }
 
-#ifdef CONFIG_IOMMU_IOVA
+#if IS_ENABLED(CONFIG_IOMMU_IOVA)
 int iova_cache_get(void);
 void iova_cache_put(void);