xserver_xorg-server: fix build on AArch64

Fixes the following build issue:

Making all in fb
  CC     libfb_la-fb24_32.lo
In file included from fb24_32.c:30:0:
fb.h:98:2: error: #error "GLYPHPADBYTES must be 4"

See:

 http://autobuild.buildroot.org/results/9135238abcde29918f8cb61002f1fb9279884a47/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>