platform: arduino: move json alloc into .cpp

src/platform/arduino/MQTTTelemetry.cpp

@@ -6,6 +6,8 @@
 #include "../../Config.h"
 #include "../../Platform.h"
 
+StaticJsonDocument<1024> m_json;
+
 struct MQTTDevice {
   const String id;
   const String name;

src/platform/arduino/MQTTTelemetry.h

@@ -12,8 +12,6 @@
 #include <WiFi.h>
 #endif
 
-#include <ArduinoJson.h>
-
 class MQTTTelemetry : public BufferedInputSource, OnlineTaskMixin {
   public:
     MQTTTelemetry();
@@ -75,5 +73,4 @@ class MQTTTelemetry : public BufferedInputSource, OnlineTaskMixin {
     WiFiClient m_wifi;
     PubSubClient m_mqtt;
     LogPrinter m_logPrinter;
-    StaticJsonDocument<1024> m_json;
 };