[json] Add forward definition header (#5676)

simulation/halsim_xrp/src/main/native/include/HALSimXRP.h

@@ -12,6 +12,7 @@
 #include <HALSimBaseWebSocketConnection.h>
 #include <WSProviderContainer.h>
 #include <WSProvider_SimDevice.h>
+#include <wpi/json_fwd.h>
 #include <wpinet/uv/Async.h>
 #include <wpinet/uv/Buffer.h>
 #include <wpinet/uv/Loop.h>
@@ -20,10 +21,6 @@
 
 #include "XRP.h"
 
-namespace wpi {
-class json;
-}  // namespace wpi
-
 namespace wpilibxrp {
 
 // This masquerades as a "WebSocket" so that we can reuse the