Seems there is a trivial typo in json.h
diff --git a/src/include/utils/json.h b/src/include/utils/json.h
index 415787b..fa0d4fc 100644
--- a/src/include/utils/json.h
+++ b/src/include/utils/json.h
@@ -26,4 +26,4 @@ extern Datum row_to_json(PG_FUNCTION_ARGS);
extern Datum row_to_json_pretty(PG_FUNCTION_ARGS);
extern void escape_json(StringInfo buf, const char *str);
-#endif /* XML_H */
+#endif /* JSON_H */