CVar: std::move listeners within addListener()

std::function is allowed to heap allocate in order to hold any necessary
captures, so we should be std::move-ing instances here in order to avoid
potentially performing reallocations.

hecl/include/hecl/CVar.hpp

@@ -117,7 +117,7 @@ public:
    */
   void lock();
 
-  void addListener(ListenerFunc func) { m_listeners.push_back(func); }
+  void addListener(ListenerFunc func) { m_listeners.push_back(std::move(func)); }
 
 private:
   void dispatch();