[json] Add forward definition header (#5676)

simulation/halsim_xrp/src/main/native/cpp/XRP.cpp

@@ -7,6 +7,7 @@
 #include <fmt/format.h>
 #include <wpi/Endian.h>
 #include <wpi/MathExtras.h>
+#include <wpi/json.h>
 
 using namespace wpilibxrp;
 

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

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

@@ -9,7 +9,7 @@
 #include <span>
 #include <string>
 
-#include <wpi/json.h>
+#include <wpi/json_fwd.h>
 #include <wpinet/raw_uv_ostream.h>
 
 #define XRP_TAG_MOTOR 0x12