glx: fix glvnd pointer types

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110709


Fixes: 22a9e00a ("glx: Implement the libglvnd interface.")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 65b016b1)