Let it build with poppler-23.10.

Index: qpdfview.pri
--- qpdfview.pri.orig
+++ qpdfview.pri
@@ -12,4 +12,4 @@ isEmpty(APP_DIR_DATA_PATH):APP_DIR_DATA_PATH = data
 win32:include(qpdfview_win32.pri)
 os2:include(qpdfview_os2.pri)
 
-CONFIG += c++11
+CONFIG += c++17