model: Some compilers got confused with "typedef policy policy". Renaming the typedefed name to policy_base

Closes #15
diff --git a/utils/trie/freshness-policy.h b/utils/trie/freshness-policy.h
index eec8400..e123d73 100644
--- a/utils/trie/freshness-policy.h
+++ b/utils/trie/freshness-policy.h
@@ -84,7 +84,7 @@
     class type : public policy_container
     {
     public:
-      typedef policy policy; // to get access to get_freshness methods from outside
+      typedef policy policy_base; // to get access to get_freshness methods from outside
       typedef Container parent_trie;
 
       type (Base &base)