Fix StelTranslator's destructure being non-virtual

Now that it has a derived StelSkyTranslator that's being deleted by base
class pointer, the destructor must be virtual.

src/core/StelTranslator.hpp

@@ -63,7 +63,7 @@ class StelTranslator
 	//! @param alangName The C locale name or language name like "fr" or "fr_FR". If string is "" or "system" it will use the system locale.
 	StelTranslator(const QString& adomain, const QString& alangName);
 
-	~StelTranslator();
+	virtual ~StelTranslator();
 
 	//! Checks whether any translations have been loaded
 	virtual bool isEmpty() const;