Our array here really is fixed, but it's size is determined by a variable
(that's assigned a fix values), unncessarily making it a VLA (even making
it const doesn't change that), so giving false alarms when compiling w/
-Wvla or -Werror=vla.
Replacing the variable by #define trivially fixes this.
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1665>
(cherry picked from commit 0a7763bfce)