builtins.toJSON: fix __toString usage

(cherry picked from commit e583df52800b4baa1564b027fe3b83a21756c2cc)

src/libexpr/eval.hh

@@ -9,6 +9,7 @@
 #include "function-trace.hh"
 
 #include <map>
+#include <optional>
 #include <unordered_map>
 
 
@@ -196,6 +197,9 @@ public:
        set with attribute `type = "derivation"'). */
     bool isDerivation(Value & v);
 
+    std::optional<string> tryAttrsToString(const Pos & pos, Value & v,
+        PathSet & context, bool coerceMore = false, bool copyToStore = true);
+
     /* String coercion.  Converts strings, paths and derivations to a
        string.  If `coerceMore' is set, also converts nulls, integers,
        booleans and lists to a string.  If `copyToStore' is set,