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

src/guiTextInputMenu.h

@@ -28,6 +28,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 struct TextDest
 {
 	virtual void gotText(std::wstring text) = 0;
+	virtual ~TextDest() {};
 };
 
 class GUITextInputMenu : public GUIModalMenu

src/inventory.h

@@ -559,6 +559,7 @@ struct InventoryAction
 	virtual void serialize(std::ostream &os) const = 0;
 	virtual void apply(InventoryContext *c, InventoryManager *mgr,
 			ServerEnvironment *env) = 0;
+	virtual ~InventoryAction() {};
 };
 
 struct IMoveAction : public InventoryAction