Remove some designated initializers

With the switch to C++20, the rules became more strict, and we can no
longer initialize base classes. Make them comments instead.

(BTW
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2287r1.html
this offers some new syntax for this use-case. Hopefully this will be
adopted and we can eventually use it.)

src/libfetchers/fetchers.cc

@@ -215,7 +215,7 @@ StorePath Input::computeStorePath(Store & store) const
             .method = FileIngestionMethod::Recursive,
             .hash = *narHash,
         },
-        .references = {},
+        /* .references = */ {},
     });
 }