fix xml declaration for logs
I've noticed
.gnome.Polari/data/TpLogger/logs/idle_irc_mclasen0/chatrooms/#gnome-boxes/20210522.log:126: parser error : xmlParseCharRef: invalid xmlChar value 7
/!\ JOIN #LIBERIA TODAY. 
^
in my logs. After some discussion, we concluded that this is due to polari using gmarkup to escape text, which produces xml 1.1, and tplogger putting version 1.0 in the xml declaration of the log files. This should be easy to fix by just changing that to 1.1.