make libnetwork compile on freebsd

Signed-off-by: Alexey Guskov <lexag@mail.ru>

drivers_freebsd.go

@@ -0,0 +1,21 @@
+package libnetwork
+
+import (
+	"github.com/docker/libnetwork/driverapi"
+	"github.com/docker/libnetwork/drivers/null"
+	"github.com/docker/libnetwork/drivers/remote"
+)
+
+type driverTable map[string]driverapi.Driver
+
+func initDrivers(dc driverapi.DriverCallback) error {
+	for _, fn := range [](func(driverapi.DriverCallback) error){
+		null.Init,
+		remote.Init,
+	} {
+		if err := fn(dc); err != nil {
+			return err
+		}
+	}
+	return nil
+}