glx: fix a macro being invoked with the wrong parameter name

glx: fix a macro being invoked with the wrong parameter name

Strangely, this was somehow compiling with GCC but my futile efforts to
build mesa with msan caused me to find clang refusing to compile because
of this. Unknown how many bugs this could fix or how GCC did manage to
find "config" in scope but it's fairly obvious that this is the correct
parameter that should be used.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20830>