Fix FreeBSD build: replace cpuset_getdomain with cpuset_getaffinity for NUMA domain CPU lookup

[Copilot]

cpuset_getdomain expects a domainset_t * as its 5th argument, but the code was passing a cpuset_t *, causing a compilation failure on FreeBSD.

Changes

• src/cpucounters.cpp: Replace cpuset_getdomain(CPU_LEVEL_WHICH, CPU_WHICH_DOMAIN, ..., &cpuset, DOMAINSET_POLICY_PREFER) with cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_DOMAIN, ..., &cpuset), which correctly accepts cpuset_t * and returns the set of CPUs belonging to a given NUMA domain.
• Remove unused #include <sys/domainset.h>.

// Before (broken)
cpuset_t cpuset;
cpuset_getdomain(CPU_LEVEL_WHICH, CPU_WHICH_DOMAIN, numa_node_id,
                 sizeof(cpuset), &cpuset, DOMAINSET_POLICY_PREFER);

// After (correct)
cpuset_t cpuset;
cpuset_getaffinity(CPU_LEVEL_WHICH, CPU_WHICH_DOMAIN, numa_node_id,
                   sizeof(cpuset), &cpuset);

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