fix: build error while running client/X11/generate_argument_docbook

Author: Viacheslav Dubrovsk <dubrsl@altlinux.org>
Bug: https://github.com/FreeRDP/FreeRDP/issues/1481
Signed-off-by: Philippe Coval <philippe.coval@open.eurogiciel.org>
Merge pull request #1794 from eurogiciel/sandbox/pcoval/for-upstream

fix: build error while running client/X11/generate_argument_docbook