fix run-make-fulldeps tests

Author: mark-i-m

src/test/run-make-fulldeps/hotplug_codegen_backend/the_backend.rs

@@ -8,6 +8,7 @@ extern crate rustc_hir;
 extern crate rustc_target;
 extern crate rustc_driver;
 extern crate rustc_span;
+extern crate rustc_session;
 
 use std::any::Any;
 use std::sync::Arc;

src/test/run-make-fulldeps/issue-19371/foo.rs

@@ -4,6 +4,7 @@ extern crate rustc;
 extern crate rustc_interface;
 extern crate rustc_driver;
 extern crate rustc_span;
+extern crate rustc_session;
 
 use rustc_session::DiagnosticOutput;
 use rustc_session::config::{Input, Options,