Fix golang.org/x/sync canonical import path

The golang.org/x/sync package was vendored using the
github.com/golang/sync URL, but this is not the canonical
URL.

Because of this, vendoring failed in Moby, as it detects
these to be a duplicate import:

    vndr github.com/golang/sync
    2018/03/14 11:54:37 Collecting initial packages
    2018/03/14 11:55:00 Download dependencies
    2018/03/14 11:55:00 Failed to parse config: invalid config format: // FIXME this should be golang.org/x/sync, which is already vendored above

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

libnetwork/ipam/parallel_test.go

@@ -7,10 +7,9 @@ import (
 	"sync"
 	"testing"
 
-	"github.com/golang/sync/semaphore"
-
 	"github.com/docker/libnetwork/ipamapi"
 	"github.com/stretchr/testify/assert"
+	"golang.org/x/sync/semaphore"
 )
 
 const (

libnetwork/vendor.conf

@@ -50,6 +50,6 @@ github.com/vishvananda/netns 604eaf189ee867d8c147fafc28def2394e878d25
 golang.org/x/crypto 558b6879de74bc843225cde5686419267ff707ca
 golang.org/x/net 7dcfb8076726a3fdd9353b6b8a1f1b6be6811bd6
 golang.org/x/sys 07c182904dbd53199946ba614a412c61d3c548f5
-github.com/golang/sync fd80eb99c8f653c847d294a001bdf2a3a6f768f5
+golang.org/x/sync fd80eb99c8f653c847d294a001bdf2a3a6f768f5
 github.com/pkg/errors 839d9e913e063e28dfd0e6c7b7512793e0a48be9
 github.com/ishidawataru/sctp 07191f837fedd2f13d1ec7b5f885f0f3ec54b1cb