util: use correct type in sysctl argument

Fixes build on OpenBSD/macppc powerpc

error: incompatible pointer types passing 'int *' to parameter of type 'size_t *'
    (aka 'unsigned long *') [-Werror,-Wincompatible-pointer-types]

Fixes: 01bd21eef8 ("gallium: Import Dennis Smit cpu detection code.")
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/6511>

src/util/u_cpu_detect.c

@@ -136,7 +136,7 @@ check_os_altivec_support(void)
    int sels[2] = {CTL_MACHDEP, CPU_ALTIVEC};
 #endif
    int has_vu = 0;
-   int len = sizeof (has_vu);
+   size_t len = sizeof (has_vu);
    int err;
 
    err = sysctl(sels, 2, &has_vu, &len, NULL, 0);