Add virtual destructors to abstract classes

IRespawnInitiator and InventoryActions are abstract classes, but they
were missing a virtual destructor. Define it, even if it does nothing
and its absence most likely makes no difference other tha causing
warnings during compile.

src/guiDeathScreen.h

@@ -29,6 +29,7 @@ class IRespawnInitiator
 {
 public:
 	virtual void respawn() = 0;
+	virtual ~IRespawnInitiator() {};
 };
 
 class GUIDeathScreen : public GUIModalMenu