Add X_BYTE_ORDER to xorg-server.h

Drivers don't get to use dix-config.h, they use xorg-server.h
instead. Add X_BYTE_ORDER to that file so drivers can see the value.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>