Pull requests started failing because vendoring didn't match;
```
The result of vndr differs
D vendor/golang.org/x/sync/singleflight/singleflight.go
Please vendor your package with github.com/LK4D4/vndr.
```
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
This updates libnetwork to 8892d7537c67232591f1f3af60587e3e77e61d41 to bring in
IPAM fixes for duplicate IP addresses.
- IPAM tests (libnetwork PR 2104) (no changes in vendored files)
- Fix for Duplicate IP issues (libnetwork PR 2105)
Also bump golang/x/sync to match libnetwork (no code-changes, other
than the README being updated)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Akihiro Suda