The linux/iommu.h header uses ERR_PTR defined
in linux/err.h but doesn't include it.

Cc:joro@8bytes.org
Reviewed-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NWang YanQing <udknight@gmail.com>
Signed-off-by: NJoerg Roedel <joro@8bytes.org>