include/linux/vfio.h: Guard powerpc-specific functions with CONFIG_VFIO_SPAPR_EEH

When CONFIG_EEH=y and CONFIG_VFIO_SPAPR_EEH=n, build fails with the
following:

    drivers/vfio/pci/vfio_pci.o: In function `.vfio_pci_release':
    vfio_pci.c:(.text+0xa98): undefined reference to `.vfio_spapr_pci_eeh_release'
    drivers/vfio/pci/vfio_pci.o: In function `.vfio_pci_open':
    vfio_pci.c:(.text+0x1420): undefined reference to `.vfio_spapr_pci_eeh_open'

In this case, vfio_pci.c should use the empty definitions of
vfio_spapr_pci_eeh_open and vfio_spapr_pci_eeh_release functions.

This patch fixes it by guarding these function definitions with
CONFIG_VFIO_SPAPR_EEH, the symbol that controls whether vfio_spapr_eeh.c is
built, which is where the non-empty versions of these functions are. We need to
make use of IS_ENABLED() macro because CONFIG_VFIO_SPAPR_EEH is a tristate
option.

This issue was found during a randconfig build. Logs are here:

    http://kisskb.ellerman.id.au/kisskb/buildresult/12982362/Signed-off-by: Murilo Opsfelder Araujo <mopsfelder@gmail.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

include/linux/vfio.h

@@ -152,7 +152,7 @@ extern int vfio_set_irqs_validate_and_prepare(struct vfio_irq_set *hdr,
 					      size_t *data_size);
 
 struct pci_dev;
-#ifdef CONFIG_EEH
+#if IS_ENABLED(CONFIG_VFIO_SPAPR_EEH)
 extern void vfio_spapr_pci_eeh_open(struct pci_dev *pdev);
 extern void vfio_spapr_pci_eeh_release(struct pci_dev *pdev);
 extern long vfio_spapr_iommu_eeh_ioctl(struct iommu_group *group,
@@ -173,7 +173,7 @@ static inline long vfio_spapr_iommu_eeh_ioctl(struct iommu_group *group,
 {
 	return -ENOTTY;
 }
-#endif /* CONFIG_EEH */
+#endif /* CONFIG_VFIO_SPAPR_EEH */
 
 /*
  * IRQfd - generic