Don't cast a string from ascii

Change-Id: Ia902e5be3ff61057b2e895a491bb68f1b7c6dd64
Reviewed-on: http://codereview.qt.nokia.com/1221
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Liang Qi <liang.qi@nokia.com>

src/gui/text/qtextdocument.cpp

@@ -2710,7 +2710,7 @@ void QTextHtmlExporter::emitBlock(const QTextBlock &block)
 
     html += QLatin1Char('>');
     if (block.begin().atEnd())
-        html += "<br />";
+        html += QLatin1String("<br />");
 
     QTextBlock::Iterator it = block.begin();
     if (fragmentMarkers && !it.atEnd() && block == doc->begin())