pacakge/freerdp: disable building the manpages

Building the manpages requires xsltproc, which might not be available.
Also, who needs the manpages on the target anyway? ;-)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>