extensions: goat-exercise appdata depends on po files.

Make sure we regenerate the appdata file when po files are updated. It
should not be too much of a problem for release builds made from
scratch, but on incremental dev builds, the appdata was not regenerated
even when new translation got added.

extensions/goat-exercises/meson.build

@@ -108,6 +108,7 @@ appdatafilein = configure_file(
 appdatafile = custom_target(appdatafilename,
   input : [ appdatafilein, ],
   output: [ appdatafilename, ],
+  depend_files: [ po_plug_ins_dir ],
   command: [
     intltool_merge,
     po_plug_ins_dir,