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/libstore/derivations.cc

@@ -7,11 +7,12 @@
 #include "split.hh"
 #include "common-protocol.hh"
 #include "common-protocol-impl.hh"
+#include "strings-inline.hh"
+#include "json-utils.hh"
+
 #include <boost/container/small_vector.hpp>
 #include <nlohmann/json.hpp>
 
-#include "strings-inline.hh"
-
 namespace nix {
 
 std::optional<StorePath> DerivationOutput::path(const StoreDirConfig & store, std::string_view drvName, OutputNameView outputName) const

src/libstore/local-store.cc

@@ -50,6 +50,8 @@
 
 #include <sqlite3.h>
 
+#include <nlohmann/json.hpp>
+
 #include "strings.hh"
 
 

src/libstore/nar-info.cc

@@ -2,6 +2,7 @@
 #include "nar-info.hh"
 #include "store-api.hh"
 #include "strings.hh"
+#include "json-utils.hh"
 
 namespace nix {