S.a. commit 8dbe0af7 which already
did the same for Gtk3KDE5FilePicker.

Change-Id: I35f837b7b8fdaebc5625ff8ea5e20b3f48a3b4ec
Reviewed-on: https://gerrit.libreoffice.org/70519
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>