add missing FIXME(const-hack)

Author: RalfJung

library/core/src/ptr/alignment.rs

@@ -155,7 +155,7 @@ impl Alignment {
  !(unsafe{self.as_usize().unchecked_sub(1)})
 }
 
-// Remove me once `Ord::max` is usable in const
+// FIXME(const-hack) Remove me once `Ord::max` is usable in const
 pub(crate)constfnmax(a:Self,b:Self) -> Self{
 if a.as_usize() > b.as_usize(){ a }else{ b }
 }