drivers/vfio: Export vfio_spapr_iommu_eeh_ioctl() with GPL

The function should have been exported with EXPORT_SYMBOL_GPL()
as part of commit 92d18a68 ("drivers/vfio: Fix EEH build error").

Suggested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>