Fix opening epub files with non ascii names on Windows

epub_open wants an utf8 in Windows

It probably also wants utf8 in Linux, but the qPrintable already gives
us an utf8 in most of the normal Linux locales, so don't touch that

BUGS: 448274

generators/epub/epubdocument.cpp

@@ -18,7 +18,11 @@ EpubDocument::EpubDocument(const QString &fileName, const QFont &font)
     , padding(20)
     , mFont(font)
 {
+#ifdef Q_OS_WIN
+    mEpub = epub_open(fileName.toUtf8(), 2);
+#else
     mEpub = epub_open(qPrintable(fileName), 2);
+#endif
 
     setPageSize(QSizeF(600, 800));
 }