Add regression test for rust-lang#56098

Author: apekros

tests/ui/foreign/foreign-fn-linkname.rs

@@ -2,6 +2,9 @@
 // ignore-wasm32-bare no libc to test ffi with
 // ignore-sgx no libc
 
+// Ensure no false positive on "unused extern crate" lint
+#![deny(unused_extern_crates)]
+
 #![feature(rustc_private)]
 
 extern crate libc;