Add #![allow(improper_ctypes)] to extern-pass-empty.rs; note this t…

…est seems bogus.

that is, I suspect it should be retired because it is testing behavior
(namely a `#[repr(C)] struct Empty;`) that, if I understand correctly,
is undefined.

See also rust-lang#53859.

src/test/ui/run-pass/extern/extern-pass-empty.rs

@@ -9,6 +9,8 @@
 // except according to those terms.
 
 // run-pass
+#![allow(improper_ctypes)] // FIXME: this test is inherently not FFI-safe.
+
 // Test a foreign function that accepts empty struct.
 
 // pretty-expanded FIXME #23616