Move M260_M261 to cpp

Marlin/src/Marlin.h

@@ -143,6 +143,11 @@ void manage_inactivity(bool ignore_stepper_queue = false);
 
 #endif // !MIXING_EXTRUDER
 
+#if ENABLED(EXPERIMENTAL_I2CBUS)
+  #include "feature/twibus.h"
+  extern TWIBus i2c;
+#endif
+
 #if ENABLED(G38_PROBE_TARGET)
   extern bool G38_move,        // flag to tell the interrupt handler that a G38 command is being run
               G38_endstop_hit; // flag from the interrupt handler to indicate if the endstop went active