PDF: fix build with PoDoFo >= 0.10 with C++20 compilation (fixes #9875)

frmts/pdf/pdfsdk_headers.h

@@ -104,6 +104,13 @@
 #endif
 
 #ifdef USE_HACK_BECAUSE_PdfInputStream_constructor_is_not_exported_in_podofo_0_11
+// If we <sstream> is included after our below #define private public errors out
+// with an error like:
+// /usr/include/c++/13.2.1/sstream:457:7: error: 'struct std::__cxx11::basic_stringbuf<_CharT, _Traits, _Alloc>::__xfer_bufptrs' redeclared with different access
+//  457 |       struct __xfer_bufptrs
+// so include it before, as otherwise it would get indirectly included by
+// PdfDate.h, which includes <chrono>, which includes <sstream>
+#include <sstream>
 // Ugly! PfdObjectStream::GetParent() is private but we need it...
 #define private public
 #endif