xserver_xorg-server: Fix race condition when installing man pages

Reproduced the bug using BR2_JLEVEL=16

Fixes race condition by disabling parallel make in man/

http://autobuild.buildroot.net/results/11e/11ece737078ca8da2fee4249432148675b4e630e/
http://autobuild.buildroot.net/results/a95/a952932b832ee3296d6c225c7a9df408ca3eef9f/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>