Add missing generic

Author: Xavientois

library/std/src/io/mod.rs

@@ -2490,7 +2490,7 @@ trait SizeHint {
 }
 
 #[stable(feature = "bufreader_size_hint", since = "1.51.0")]
-impl SizeHint for T {
+impl<T> SizeHint for T {
     fn lower_bound(&self) -> usize {
         0
     }