Fix compilation after b0afad8f0b

That commit made QString::toXxx (8-bit) functions use C++11 ref
qualifiers, so we need to match it here.

Change-Id: I45b50464d36f858d012b12e0cb511aae347ddb6f
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

tests/auto/corelib/codecs/utf8/tst_utf8.cpp

@@ -53,7 +53,11 @@ public:
     // test data:
     QTextCodec *codec;
     QString (*from8BitPtr)(const char *, int);
+#ifdef Q_COMPILER_REF_QUALIFIERS
+    QByteArray (QString:: *to8Bit)() const &;
+#else
     QByteArray (QString:: *to8Bit)() const;
+#endif
 
     inline QString from8Bit(const QByteArray &ba)
     { return from8BitPtr(ba.constData(), ba.length()); }