fix linking of postcard test

crates/proc-macro-srv-cli/tests/bidirectional_postcard.rs

diff --git a/crates/proc-macro-srv-cli/tests/bidirectional_postcard.rs b/crates/proc-macro-srv-cli/tests/bidirectional_postcard.rs
index 33ca1d7..ba9657a 100644
--- a/crates/proc-macro-srv-cli/tests/bidirectional_postcard.rs
+++ b/crates/proc-macro-srv-cli/tests/bidirectional_postcard.rs
@@ -1,4 +1,8 @@
 #![cfg(feature = "sysroot-abi")]
+#![cfg_attr(feature = "in-rust-tree", feature(rustc_private))]
+
+#[cfg(feature = "in-rust-tree")]
+extern crate rustc_driver as _;
 
 mod common {
     pub(crate) mod utils;