QDateTime: early return to make a Q_UNREACHABLE actually true.

It could be reached when QT_BOOTSTRAPPED was defined.

Change-Id: I632d6f908a3bcbde81a6ebbadcaf2800dfe1449d
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qdatetime.cpp

@@ -2698,7 +2698,7 @@ qint64 QDateTimePrivate::toMSecsSinceEpoch() const
 
     case Qt::TimeZone:
 #ifdef QT_BOOTSTRAPPED
-        break;
+        return 0;
 #else
         return zoneMSecsToEpochMSecs(m_msecs, m_timeZone);
 #endif