Resolve compiler warnings for unused return values

* Fixes #1932 - See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425#c29

Adding a negation before the function call allows the (void) syntax to work properly.

tests/gui/TestGui.h

@@ -72,7 +72,7 @@ private slots:
     void testTrayRestoreHide();
 
 private:
-    int addCannedEntries();
+    void addCannedEntries();
     void checkDatabase(QString dbFileName = "");
     void triggerAction(const QString& name);
     void dragAndDropGroup(const QModelIndex& sourceIndex,