Signposts generated by perf_event(PERF_EVENT_SIGNPOST) now show up in
profile timelines, and if you hover them you get a tooltip with the two
arguments passed with the event.
While structs being forward declared as classes is not strictly an
issue, Clang complains as this is not portable code, since some ABIs
treat classes declared as `class` and `struct` differently.
It's easier to fix these than to reason about explicitly disabling
another warning.