Future proof an unstable test

statics (even extern ones) will get their type checked for a `Sized` bound in order to fix rust-lang/rust#54410

Author: oli-obk

tests/testsuite/doc.rs

@@ -571,6 +571,9 @@ fn doc_target() {
             #![feature(no_core)]
             #![no_core]
 
+            #[lang = "sized"]
+            trait Sized {}
+
             extern {
                 pub static A: u32;
             }