xsfbs.mk: fix the prepare target for parallel make
xsfbs.mk: fix the prepare target for parallel make

philipn/xserver-xorg-input-synaptics,facboy/xserver-xorg-video-qxl,philipn/xserver-xorg-input-synaptics,RsrchBoy/xserver-xorg-input-synaptics,facboy/xserver-xorg-video-qxl,RsrchBoy/xserver-xorg-input-synaptics,philipn/xserver-xorg-input-synaptics,facboy/xserver-xorg-video-qxl,facboy/xserver-xorg-video-qxl