structcheck: nolint for false positives

```
builder/fscache/fscache.go:505:2: `src` is unused (structcheck)
builder/fscache/fscache.go:507:2: `cached` is unused (structcheck)

daemon/graphdriver/vfs/quota_linux.go:9:2: `quotaCtl` is unused (structcheck)
daemon/graphdriver/vfs/quota_linux.go:10:2: `quotaOpt` is unused (structcheck)
```

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

builder/fscache/fscache.go

@@ -497,6 +497,7 @@ type sourceMeta struct {
 	Size        int64
 }
 
+//nolint:structcheck
 type cachedSource struct {
 	sourceMeta
 	refs    map[*cachedSourceRef]struct{}

daemon/graphdriver/vfs/quota_linux.go

@@ -5,6 +5,7 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+//nolint:structcheck
 type driverQuota struct {
 	quotaCtl *quota.Control
 	quotaOpt quota.Quota