um: virtio_uml: include linux/virtio-uml.h

This fixes a sparse warning, since the function defined
here should have a declaration (or be static).

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 43c590cb ("um: virtio/pci: enable suspend/resume")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>