Restored the previously suppressed __attribute__ unused in a portable way

plugins/samplesource/kiwisdr/kiwisdrplugin.cpp

@@ -79,6 +79,9 @@ PluginInstanceGUI* KiwiSDRPlugin::createSampleSourcePluginInstanceGUI(
         QWidget **widget,
         DeviceUISet *deviceUISet)
 {
+    (void) sourceId;
+    (void) widget;
+    (void) deviceUISet;
     return 0;
 }
 #else