requires virtual destructor

Index: effects.h
--- effects.h.orig
+++ effects.h
@@ -33,6 +33,7 @@ class SpecialEffect (protected)
 public:
     SpecialEffect(NjamFont *f) { fontM = f; };
     virtual bool Update() = 0;
+    virtual ~SpecialEffect();
 };
 //-----------------------------------------------------------------------------
 class Effects
