BUILD: config: avoid a build warning on numa_detect_topology() without threads

BUILD: config: avoid a build warning on numa_detect_topology() without threads

The function is defined when using linux+cpu affinity but is only used
if threads are enabled, so let's add this condition to avoid aa build
warning about an unused function when building with thread disabled.
This came in 2.4-dev17 with commit b56a7c89a ("MEDIUM: cfgparse: detect
numa and set affinity if needed") so no backport is needed.