fix broken reference to Instant in pal

Author: max-niederman

library/std/src/sys/pal/vexos/thread.rs

@@ -2,7 +2,7 @@ use super::unsupported;
 use crate::ffi::CStr;
 use crate::io;
 use crate::num::NonZero;
-use crate::time::Duration;
+use crate::time::{Duration, Instant};
 
 pub struct Thread(!);