Use explicit re-export of serde_derive to give rustc more info

rustc will start looking behind `#[cfg(FALSE)]` items to start giving
better diagnostics. By using an explicit re-export instead of a glob
export, we tell rustc that `Deserialize` and `Serialize` exist here.

serde/src/lib.rs

@@ -340,7 +340,7 @@ mod std_error;
 extern crate serde_derive;
 #[cfg(feature = "serde_derive")]
 #[doc(hidden)]
-pub use serde_derive::*;
+pub use serde_derive::{Deserialize, Serialize};
 
 #[cfg(all(not(no_serde_derive), any(feature = "std", feature = "alloc")))]
 mod actually_private {