Fix broken build related to missing xmlto dependency used to generate
xfreerdp(1) man page.

> Building package for freerdp-2.0.0.r1

pkg-static: Unable to access file /wrkdirs/usr/ports/net/freerdp/work/stage/usr/local/man/man1/xfreerdp.1.gz:No such file or directory

Reported by: Andrzej Tobola <ato@iem.pw.edu.pl> via email
Approved by: kevans (maintainer)