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/store-api.cc

@@ -234,7 +234,7 @@ std::pair<StorePath, Hash> Store::computeStorePathForPath(std::string_view name,
             .method = method,
             .hash = h,
         },
-        .references = {},
+        /* .references = */ {},
     };
     return std::make_pair(makeFixedOutputPath(name, caInfo), h);
 }
@@ -443,7 +443,7 @@ ValidPathInfo Store::addToStoreSlow(std::string_view name, const Path & srcPath,
                 .method = method,
                 .hash = hash,
             },
-            .references = {},
+            /* .references = */ {},
         },
         narHash,
     };