hurd: Add domainname field to utsname

158cd30 ("hurd: fix definition of utsname struct") dropped
it saying it is unused, but it is still there and read by the
platform-info crate, whose build thus got broken. It is less
troublesome to just declare the field.

src/unix/hurd/mod.rs

@@ -876,6 +876,7 @@ s! {
         pub release: [::c_char; _UTSNAME_LENGTH],
         pub version: [::c_char; _UTSNAME_LENGTH],
         pub machine: [::c_char; _UTSNAME_LENGTH],
+        pub domainname: [::c_char; _UTSNAME_LENGTH],
     }
 
     pub struct rlimit64 {