constify Default on Nanoseconds

library/core/src/num/niche_types.rs

@@ -112,7 +112,8 @@ impl Nanoseconds {
     pub const ZERO: Self = unsafe { Nanoseconds::new_unchecked(0) };
 }
 
-impl Default for Nanoseconds {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+impl const Default for Nanoseconds {
     #[inline]
     fn default() -> Self {
         Self::ZERO