auto-update: make restarted unit more obvious

The output of auto-update leaves quite some space for improvements.  One
thing is to make it more obvious which systemd units were restarted.

With this change, the output looks as follows:
```
$ podman auto-update
Trying to pull...

Restarted the following systemd units:
$unit-1
$unit-2
$unit-3
```

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/cmd/podman/auto-update.go b/cmd/podman/auto-update.go
index 99226790f..34cc7d74b 100644
--- a/cmd/podman/auto-update.go
+++ b/cmd/podman/auto-update.go
@@ -50,7 +50,9 @@ func autoUpdate(cmd *cobra.Command, args []string) error {
 		return errors.Errorf("`%s` takes no arguments", cmd.CommandPath())
 	}
 	report, failures := registry.ContainerEngine().AutoUpdate(registry.GetContext(), autoUpdateOptions)
-	if report != nil {
+	if report != nil && len(report.Units) > 0 {
+		// Make it more obvious to users what the output means.
+		fmt.Println("\nRestarted the following systemd units:")
 		for _, unit := range report.Units {
 			fmt.Println(unit)
 		}