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