Fix Fuchsia build broken by rust-lang#124210

Fuchsia doesn't support dirfd although we have a symbol stubbed for it.

Author: David Koloski

Diff for: std/src/sys/pal/unix/fs.rs

@@ -894,6 +894,7 @@ impl Drop for Dir {
             target_os = "vita",
             target_os = "hurd",
             target_os = "espidf",
+            target_os = "fuchsia",
         )))]
         {
             let fd = unsafe { libc::dirfd(self.0) };