Update to c/image v4.0.1

Update to use the correct c/image/v4 import path, work originally from
https://github.com/containers/skopeo/pull/733 by Valentin Rothberg <rothberg@redhat.com>.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/github.com/containers/libtrust/ec_key_openssl.go

@@ -0,0 +1,24 @@
+// +build libtrust_openssl
+
+package libtrust
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/rand"
+	"fmt"
+	"io"
+	"math/big"
+)
+
+func (k *ecPrivateKey) sign(data io.Reader, hashID crypto.Hash) (r, s *big.Int, err error) {
+	hId := k.signatureAlgorithm.HashID()
+	buf := new(bytes.Buffer)
+	_, err = buf.ReadFrom(data)
+	if err != nil {
+		return nil, nil, fmt.Errorf("error reading data: %s", err)
+	}
+
+	return ecdsa.HashSign(rand.Reader, k.PrivateKey, buf.Bytes(), hId)
+}