experimental-features.hh: Don't include json-utils.hh

This caused nlohmann/json.hpp to leak into a lot of compilation units,
which is slow (when not using precompiled headers).

Cuts build time from 46m24s to 42m5s (real time with -j24: 2m42s to
2m24s).

src/libutil/config-global.cc

@@ -1,5 +1,7 @@
 #include "config-global.hh"
 
+#include <nlohmann/json.hpp>
+
 namespace nix {
 
 bool GlobalConfig::set(const std::string & name, const std::string & value)

src/libutil/experimental-features.hh

@@ -2,9 +2,10 @@
 ///@file
 
 #include "error.hh"
-#include "json-utils.hh"
 #include "types.hh"
 
+#include <nlohmann/json_fwd.hpp>
+
 namespace nix {
 
 /**
@@ -98,10 +99,4 @@ public:
 void to_json(nlohmann::json &, const ExperimentalFeature &);
 void from_json(const nlohmann::json &, ExperimentalFeature &);
 
-/**
- * It is always rendered as a string
- */
-template<>
-struct json_avoids_null<ExperimentalFeature> : std::true_type {};
-
 }

src/libutil/json-utils.hh

@@ -3,12 +3,13 @@
 
 #include <nlohmann/json.hpp>
 #include <list>
-#include <nlohmann/json_fwd.hpp>
 
 #include "types.hh"
 
 namespace nix {
 
+enum struct ExperimentalFeature;
+
 const nlohmann::json * get(const nlohmann::json & map, const std::string & key);
 
 nlohmann::json * get(nlohmann::json & map, const std::string & key);
@@ -71,6 +72,12 @@ struct json_avoids_null<std::list<T>> : std::true_type {};
 template<typename K, typename V>
 struct json_avoids_null<std::map<K, V>> : std::true_type {};
 
+/**
+ * `ExperimentalFeature` is always rendered as a string.
+ */
+template<>
+struct json_avoids_null<ExperimentalFeature> : std::true_type {};
+
 }
 
 namespace nlohmann {