util: fix util_cpu_detect_once() build on OpenBSD

util: fix util_cpu_detect_once() build on OpenBSD

Correct type for sysctl argument to fix the build.

../src/util/u_cpu_detect.c:631:29: error: incompatible pointer types passing 'int *' to parameter of type 'size_t *' (aka 'unsigned long *') [-Werror,-Wincompatible-pointer-types]
      sysctl(mib, 2, &ncpu, &len, NULL, 0);
                            ^~~~

Fixes: 5623c75e40b ("util: Fix setting nr_cpus on some BSD variants")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13448>