libcontainer: configs: extend unsupported os

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

libcontainer/configs/namespaces_unsupported.go

@@ -0,0 +1,8 @@
+// +build !linux,!freebsd
+
+package configs
+
+// Namespace defines configuration for each namespace.  It specifies an
+// alternate path that is able to be joined via setns.
+type Namespace struct {
+}