iommu/iova: Fix module config properly

IOMMU_IOVA is intended to be an optional library for users to select as
and when they desire. Since it can be a module now, this means that
built-in code which has chosen not to select it should not fail to link
if it happens to have selected as a module by someone else. Replace
IS_ENABLED() with IS_REACHABLE() to do the right thing.

CC: Thierry Reding <[e-mail]>
Reported-by: John Garry <[e-mail]>
Fixes: 15bbdec3931e ("iommu: Make the iova library a module")
Signed-off-by: Robin Murphy <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Link: https://lore.kernel.org/r/548c[e-mail]
Signed-off-by: Joerg Roedel <[e-mail]>

include/linux/iova.h

···
 	return iova >> iova_shift(iovad);
 }
 
-#if IS_ENABLED(CONFIG_IOMMU_IOVA)
+#if IS_REACHABLE(CONFIG_IOMMU_IOVA)
 int iova_cache_get(void);
 void iova_cache_put(void);