QStringBuilder: make the nested ConvertTo typedef public

It's needed by QStringTokenizer, and it's more generally useful as a
public type (you don't need to match the type of the conversion
operator, but can just access the nested typedef).

Change-Id: I2cf8b22f1ca31c38d51d2143932115da6aefc5a7
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/text/qstringbuilder.h

@@ -126,8 +126,8 @@ private:
     }
 
     typedef QConcatenable<QStringBuilder<A, B> > Concatenable;
-    typedef typename Concatenable::ConvertTo ConvertTo;
 public:
+    typedef typename Concatenable::ConvertTo ConvertTo;
     operator ConvertTo() const { return convertTo<ConvertTo>(); }
 
     int size() const { return Concatenable::size(*this); }