Mark code as ignored due to requiring external plugin

Author: midgleyc

compiler/rustc_error_codes/src/error_codes/E0498.md

@@ -12,7 +12,7 @@ The `#[plugin]` attribute should take a single argument: the name of the plugin.
 
 For example, for the plugin `foo`:
 
-```
+```ignore (requires external plugin crate)
 #![feature(plugin)]
 #![plugin(foo)] // ok!
 ```