Fix new-delete-type-mismatch in ringdht/p2p

Do not delete an object of derived class type through a pointer to
its base class type that has a non-virtual destructor. Instead, the
base class should be defined with a virtual destructor. Deleting an
object through a pointer to a type without a virtual destructor
results in undefined behavior.

Change-Id: I5a9f1ade5d67056f9ebc2865bd3c1c17fe197fcf
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>

src/ringdht/p2p.cpp

@@ -129,6 +129,7 @@ struct CtrlMsgBase
 {
     CtrlMsgBase() = delete;
     explicit CtrlMsgBase(CtrlMsgType id) : id_ {id} {}
+    virtual ~CtrlMsgBase() = default;
     CtrlMsgType type() const noexcept { return id_; }
 private:
     const CtrlMsgType id_;