Make supervisor.go to compile without libcontainer.

Signed-off-by: David Calavera <david.calavera@gmail.com>

supervisor_unsupported.go

@@ -0,0 +1,13 @@
+// +build !libcontainer
+
+package containerd
+
+import (
+	"errors"
+
+	"github.com/docker/containerd/runtime"
+)
+
+func newRuntime(stateDir string) (runtime.Runtime, error) {
+	return nil, errors.New("Unsupported runtime")
+}