iommu: Fix IOMMU sysfs stubs

0-day kernel build testing reports:

   arch/x86/kvm/x86.o: In function `iommu_device_destroy':
>> (.text+0x7a0a): multiple definition of `iommu_device_destroy'
   arch/x86/kvm/../../../virt/kvm/vfio.o:vfio.c:(.text+0x490): first defined here
   arch/x86/kvm/x86.o: In function `iommu_device_link':
>> (.text+0x7a15): multiple definition of `iommu_device_link'
   arch/x86/kvm/../../../virt/kvm/vfio.o:vfio.c:(.text+0x49b): first defined here
   arch/x86/kvm/x86.o: In function `iommu_device_unlink':
>> (.text+0x7a25): multiple definition of `iommu_device_unlink'
   arch/x86/kvm/../../../virt/kvm/vfio.o:vfio.c:(.text+0x4ab): first defined here
   arch/x86/kvm/x86.o: In function `iommu_device_create':
>> (.text+0x79f8): multiple definition of `iommu_device_create'
   arch/x86/kvm/../../../virt/kvm/vfio.o:vfio.c:(.text+0x47e): first defined here

These are due to failing to define the stubs as static inline.  Fix.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>