Add missing rustc_private

Author: lnicola

src/tools/rust-analyzer/crates/mbe/src/lib.rs

@@ -6,6 +6,8 @@
 //! The tests for this functionality live in another crate:
 //! `hir_def::macro_expansion_tests::mbe`.
 
+#![cfg_attr(feature = "in-rust-tree", feature(rustc_private))]
+
 #[cfg(not(feature = "in-rust-tree"))]
 extern crate ra_ap_rustc_lexer as rustc_lexer;
 #[cfg(feature = "in-rust-tree")]