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/libstore/local-store.cc

@@ -1419,7 +1419,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
             .method = method,
             .hash = hash,
         },
-        .references = {
+        /* .references = */ {
             .others = references,
             .self = false,
         },