Reintroduce restart logic to nvidia-cdi-refresh.service

The restart logic was removed when a dependency on the multi-user-target was added in
https://github.com/NVIDIA/nvidia-container-toolkit/commit/5fe6b42eab11f144866de07df4f755845c4b94cf.
However, this change had unforeseen consequences and was later reverted in
https://github.com/NVIDIA/nvidia-container-toolkit/commit/5eee5ce7b8ab19012f5aabdafd525f21b3a54496.
When reverting this change, the restart logic was not re-added to the service.

Signed-off-by: Christopher Desiniotis <cdesiniotis@nvidia.com>