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>