The comment about ClientPtr also being defined in misc.h is outdated
for about 20 years now: XFree86 4.3.0.1 dropped the redundant definition
over there.
It's still also in Xdefs.h, so the guard is still needed.
Fixes: d568221710
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1514>