Don't include nlohmann/json.hpp in config.hh

Instead make a separate header with the template implementation of
`BaseSetting<T>::toJSONObj` that can be included where needed

src/libstore/globals.hh

@@ -2,6 +2,7 @@
 
 #include "types.hh"
 #include "config.hh"
+#include "abstractsettingtojson.hh"
 #include "util.hh"
 
 #include <map>

src/libutil/abstractsettingtojson.hh

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <nlohmann/json.hpp>
+#include "config.hh"
+
+namespace nix {
+template<typename T>
+std::map<std::string, nlohmann::json> BaseSetting<T>::toJSONObject()
+{
+    auto obj = AbstractSetting::toJSONObject();
+    obj.emplace("value", value);
+    obj.emplace("defaultValue", defaultValue);
+    return obj;
+}
+}

src/libutil/config.cc

@@ -1,5 +1,6 @@
 #include "config.hh"
 #include "args.hh"
+#include "abstractsettingtojson.hh"
 
 #include <nlohmann/json.hpp>
 

src/libutil/config.hh

@@ -4,7 +4,7 @@
 
 #include "types.hh"
 
-#include <nlohmann/json.hpp>
+#include <nlohmann/json_fwd.hpp>
 
 #pragma once
 
@@ -255,13 +255,7 @@ public:
 
     void convertToArg(Args & args, const std::string & category) override;
 
-    std::map<std::string, nlohmann::json> toJSONObject() override
-    {
-        auto obj = AbstractSetting::toJSONObject();
-        obj.emplace("value", value);
-        obj.emplace("defaultValue", defaultValue);
-        return obj;
-    }
+    std::map<std::string, nlohmann::json> toJSONObject() override;
 };
 
 template<typename T>