router_add_listener: use undocumented(?) alias to get mtimespec
router_add_listener: use undocumented(?) alias to get mtimespec

Turns out st_mtimespec only exists on Darwin, Linux has st_mtim, but
both have compatibility defines for st_mtime, so use that.

grobian/carbon-c-relay,grobian/carbon-c-relay,grobian/carbon-c-relay