fix "type 'char' cannot be used prior to '::'"

src/ptree.cc

@@ -80,7 +80,7 @@ void format_ptree::flush()
 
   switch (format) {
   case FORMAT_XML:
-    property_tree::xml_writer_settings<char> indented(' ', 2);
+    auto indented = property_tree::xml_writer_make_settings<std::string> (' ', 2);
     property_tree::write_xml(out, pt, indented);
     out << std::endl;
     break;