Add empty virtual destructor for lru_node_t.

history_lru_node_t has implicit destructor defined. However, because
it's being deleted as lru_node_t, it's not being actually called, as
lru_node_t doesn't have a virtual destructor.

lru.h

@@ -36,6 +36,9 @@ public:
     /** Constructor */
     lru_node_t(const wcstring &pkey) : prev(NULL), next(NULL), key(pkey) { }
 
+    /** Virtual destructor that does nothing for classes that inherit lru_node_t */
+    virtual ~lru_node_t() {}
+
     /** operator< for std::set */
     bool operator<(const lru_node_t &other) const
     {