db::commitlog: entry_writer add a virtual destructor

Add a virtual destructor for a base class commitlog::entry_writer.

Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>
Message-Id: <1498511180-18391-1-git-send-email-vladz@scylladb.com>

db/commitlog/commitlog.hh

@@ -354,6 +354,7 @@ private:
         // Returns segment-independent size of the entry. Must be <= than segment-dependant size.
         virtual size_t size() = 0;
         virtual void write(segment&, output&) = 0;
+        virtual ~entry_writer() {};
     };
 };