freerdp: disable man pages generation

Man pages generation uses xmlto, and it seems that it is currently
broken on OE.

Change-Id: I1dccb9339c20b0b898344cf60c02a705d9c32794
Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>