logger/journald: U1000: field `mu` is unused (unused)

this looks to be a false positive, but this field is not
used if journald is not supported, which may be the cause

```
daemon/logger/journald/journald.go:21:2: U1000: field `mu` is unused (unused)
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/logger/journald/journald.go

@@ -18,7 +18,7 @@ import (
 const name = "journald"
 
 type journald struct {
-	mu      sync.Mutex
+	mu      sync.Mutex        //nolint:structcheck,unused
 	vars    map[string]string // additional variables and values to send to the journal along with the log message
 	readers map[*logger.LogWatcher]struct{}
 }