[json] Add forward definition header (#5676)

wpiutil/src/main/native/thirdparty/json/include/wpi/json_fwd.h

@@ -0,0 +1,9 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+namespace wpi {
+class json;
+}  // namespace wpi