doc: Turn off special features for rustdoc tests

These were only used for the markdown tests, and there's no reason they should
be distinct from the other tests.

src/doc/guide-unsafe.md

@@ -523,6 +523,7 @@ vectors provided from C, using idiomatic Rust practices.
 
 ```
 #![no_std]
+#![feature(globs)]
 
 # extern crate libc;
 extern crate core;