wasm: Mark switch case as unreachable

Fix "non-void function does not return a value in all
control paths" warning.

Change-Id: Idc58319b2704895ec56decabbdad67ffbf633ad7
Reviewed-by: Piotr Wierciński <piotr.wiercinski@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/io/qsettings_wasm.cpp

@@ -427,7 +427,7 @@ QSettingsPrivate *QSettingsPrivate::create(QSettings::Format format, QSettings::
     case QSettings::Format::InvalidFormat:
         return nullptr;
     case QSettings::Format::NativeFormat:
-        /* NOTREACHED */ assert(0);
+        Q_UNREACHABLE();
         break;
     }
 }