pdfworker: don't use deprecated search overload

gui/pdfworker.cpp

@@ -70,7 +70,7 @@ struct Renderer
 		QImage img = pdfPage->renderToImage(scaleX, scaleY);
 		for(QString &word : wordsToHighlight)
 		{
-			QList<QRectF> rects = pdfPage->search(word, Poppler::Page::CaseInsensitive);
+			QList<QRectF> rects = pdfPage->search(word, Poppler::Page::SearchFlag::IgnoreCase);
 			for(QRectF &rect : rects)
 			{
 				QPainter painter(&img);