solver: separate linux-only files

Signed-off-by: Tibor Vass <tibor@docker.com>

solver/llbsolver/file/user_nolinux.go

@@ -0,0 +1,14 @@
+// +build !linux
+
+package file
+
+import (
+	"github.com/moby/buildkit/solver/llbsolver/ops/fileoptypes"
+	"github.com/moby/buildkit/solver/pb"
+	"github.com/pkg/errors"
+	copy "github.com/tonistiigi/fsutil/copy"
+)
+
+func readUser(chopt *pb.ChownOpt, mu, mg fileoptypes.Mount) (*copy.ChownOpt, error) {
+	return nil, errors.New("only implemented in linux")
+}