rustc_mir: Mark the crate as unstable

Wouldn't want to be able to link to this on stable Rust!

src/librustc_mir/lib.rs

@@ -18,8 +18,10 @@ Rust MIR: a lowered representation of Rust. Also: an experiment!
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]
 #![cfg_attr(not(stage0), deny(warnings))]
+#![unstable(feature = "rustc_private", issue = "27812")]
 
 #![feature(rustc_private)]
+#![feature(staged_api)]
 
 #[macro_use] extern crate log;
 extern crate graphviz as dot;