message: Fix duplicate definition of `Callsite`
  Fixes the build.
  I think GitHub messed this up while merging one of my PRs.
diff: @@ -156,7 +156,6 @@ pub mod span;
  pub mod subscriber;
  pub use self::{
  - callsite::Callsite,
  dispatcher::Dispatch,
  field::{AsValue, IntoValue, Value},
  span::{Data as SpanData, Id as SpanId, Span},
  