Add proc_macro mod (copy from lib_proc_macro)

crates/ra_proc_macro_srv/src/lib.rs

@@ -10,6 +10,10 @@
 //! * By **copying** the whole rustc `lib_proc_macro` code, we are able to build this with `stable`
 //!   rustc rather than `unstable`. (Although in gerenal ABI compatibility is still an issue)
 
+#[allow(dead_code)]
+#[doc(hidden)]
+mod proc_macro;
+
 use ra_proc_macro::{ExpansionResult, ExpansionTask, ListMacrosResult, ListMacrosTask};
 
 pub fn expand_task(_task: &ExpansionTask) -> Result<ExpansionResult, String> {