Allow dead code in thread local dtor

The dead_code lint was previously eroneously missing this dead code.
Since this lint bug has been fixed, the unused field need
to be removed or marked as `#[allow(dead_code)]`.

Given the nature of this code, I don't feel confident removing the field
so it is only marked as allow(dead_code).

Author: krtab

library/std/src/sys/unix/thread_local_dtor.rs

@@ -34,7 +34,7 @@ pub unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern "C" fn(*mut u8)) {
     #[cfg(not(sanitizer_cfi_normalize_integers))]
     #[cfi_encoding = "i"]
     #[repr(transparent)]
-    pub struct c_int(pub libc::c_int);
+    pub struct c_int(#[allow(dead_code)] pub libc::c_int);
 
     extern "C" {
         #[linkage = "extern_weak"]