Debugger: Avoid a warning in a manual test

Change-Id: I54b34434384e0decde28a4f7e1decdd51b8871c1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

tests/manual/debugger/simple/simple_test_plugin.cpp

@@ -9,6 +9,7 @@ public:
     Something() {
         int i = 0;
         ++i;
+        Q_UNUSED(i)
         return;
     }
 };