xfce.thunar: patch to avoid error

«the desktop file … is in an insecure location»
which pops up when invoking desktop files that are
symlinks to the /nix/store

this error was added by this commit:

https://github.com/xfce-mirror/thunar/commit/1ec8ff89ec5a3314fcd6a57f1475654ddecc9875

«Only allow direct execution of desktop files in an
XDG directory and if they are executable.»