Add virtual destructor.

git-svn-id: svn://localhost/ardour2/branches/3.0@6871 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/pbd/pbd/property_basics.h

@@ -82,6 +82,8 @@ public:
 		: _property_id (pid)
 	{}
 
+	virtual ~PropertyBase () {}
+
 	/** Forget about any old value for this state */
 	virtual void clear_history () = 0;