libnetwork: fix some missing imports on macOS and FreeBSD

This was introduced in 1980deffae, which
changed the implementation, but forgot to update imports in these.

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

libnetwork/drivers_freebsd.go

@@ -1,6 +1,7 @@
 package libnetwork
 
 import (
+	"github.com/docker/docker/libnetwork/driverapi"
 	"github.com/docker/docker/libnetwork/drivers/null"
 )
 

libnetwork/drivers_unsupported.go

@@ -2,6 +2,8 @@
 
 package libnetwork
 
+import "github.com/docker/docker/libnetwork/driverapi"
+
 func registerNetworkDrivers(r driverapi.Registerer, driverConfig func(string) map[string]interface{}) error {
 	return nil
 }