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.)

perl/lib/Nix/Store.xs

@@ -299,7 +299,7 @@ SV * makeFixedOutputPath(int recursive, char * algo, char * hash, char * name)
                     .method = method,
                     .hash = h,
                 },
-                .references = {},
+                /* .references = */ {},
             });
             XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(path).c_str(), 0)));
         } catch (Error & e) {