flatpak-dir: Add a debug message to remove_old_appstream_tmpdirs()

This should make it a bit clearer when `rm -rf` is being used in the
debug logs.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
(cherry picked from commit 6c7eb34dd6)

common/flatpak-dir.c

@@ -4554,6 +4554,7 @@ remove_old_appstream_tmpdirs (GFile *dir)
       tmp = g_file_get_child (dir, dent->d_name);
 
       /* We ignore errors here, no need to worry anyone */
+      g_debug ("Deleting stale appstream deploy tmpdir %s", flatpak_file_get_path_cached (tmp));
       (void)flatpak_rm_rf (tmp, NULL, NULL);
     }
 }