Include the full nlohmann/json header in config.hh

It is apparently required for using `toJSONObject()`, which we do inside
the header file (because it's in a template).

This was accidentally working when building Nix itself (presumably because
`config.hh` was always included after `nlohman/json.hpp`) but caused a
(pretty dirty) build failure in the perl bindings package.

src/libutil/config.hh

@@ -4,7 +4,7 @@
 
 #include "types.hh"
 
-#include <nlohmann/json_fwd.hpp>
+#include <nlohmann/json.hpp>
 
 #pragma once