package/x11r7/xserver_xorg-server: fix build with composite support disabled

Added upstream to fix
http://autobuild.buildroot.net/results/44e/44e29c2fb5d236a66e0466698fbc9201a37dbc63/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>