drivers/vfio: Allow EEH to be built as module

This adds necessary declarations to the SPAPR VFIO EEH module,
otherwise multiple dynamic linker errors reported:

vfio_spapr_eeh: Unknown symbol eeh_pe_set_option (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_configure (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_reset (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_get_state (err 0)
vfio_spapr_eeh: Unknown symbol eeh_iommu_group_to_pe (err 0)
vfio_spapr_eeh: Unknown symbol eeh_dev_open (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_set_option (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_configure (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_reset (err 0)
vfio_spapr_eeh: Unknown symbol eeh_pe_get_state (err 0)
vfio_spapr_eeh: Unknown symbol eeh_iommu_group_to_pe (err 0)
vfio_spapr_eeh: Unknown symbol eeh_dev_open (err 0)

Signed-off-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>