fix: build error while running client/X11/generate_argument_docbook

Patch from Viacheslav Dubrovsk

Bug: https://github.com/FreeRDP/FreeRDP/issues/1481
Change-Id: I9edbef6943b49d85e54643119f474356d63a9016
Signed-off-by: Philippe Coval <philippe.coval@open.eurogiciel.org>