thread/Thread: make the destructor non-virtual

The class does not have any virtual methods, and thus the (debug-only)
destructor does not need to be virtual.

src/thread/Thread.hxx

@@ -68,7 +68,7 @@ public:
 	Thread(const Thread &) = delete;
 
 #ifndef NDEBUG
-	virtual ~Thread() {
+	~Thread() {
 		/* all Thread objects must be destructed manually by calling
 		   Join(), to clean up */
 		assert(!IsDefined());