iommu/iova: Fix compilation error with !CONFIG_IOMMU_IOVA

The stub function for !CONFIG_IOMMU_IOVA needs to be
'static inline'.

Fixes: effa4678

 ('iommu/vt-d: Don't queue_iova() if there is no flush queue')
Signed-off-by: Joerg Roedel <jroedel@suse.de>