fix older compiler

after my commit e0f3e7c0
"tdf#42374 some small optimisations for opening this PDF file"

Change-Id: I908770ca618581ffe26ebb2205b804d1abcae5a7

sdext/source/pdfimport/tree/writertreevisiting.cxx

@@ -402,7 +402,9 @@ void WriterXmlOptimizer::visit( PolyPolyElement& elem, const std::list< Element*
         elem.Action |= pNext->Action;
 
         elem.Children.splice( elem.Children.end(), pNext->Children );
-                        elem.Parent->Children.erase( next_it );
+        // workaround older compilers that do not have std::list::erase(const_iterator)
+        std::list< Element* > tmp;
+        tmp.splice( tmp.begin(), elem.Parent->Children, next_it, next_it);
         delete pNext;
     }
 }