powersupplyclass_darwin: enable builds against older macOS SDK
powersupplyclass_darwin: enable builds against older macOS SDK

This is necessary to build on darwin using nix, as nix-darwin uses an
older macOS SDK, built from Apple's open source releases.

Signed-off-by: Peter Woodman <4b8373d016f277527198385ba72fda0feb5da015@shortbus.org>

prometheus/node_exporter,prometheus/node_exporter,prometheus/node_exporter