[intermcu] fix ABI in fbw

in every target ABI is used, ABI_C needs to be defined in one file that includes abi.h
Since main_fbw doesn't need ABI, do it in intermcu_fbw

sw/airborne/subsystems/intermcu/intermcu_fbw.c

@@ -24,6 +24,8 @@
  *  @brief Rotorcraft Inter-MCU on FlyByWire
  */
 
+#define ABI_C
+
 #include "intermcu_fbw.h"
 #include "pprzlink/intermcu_msg.h"
 #include "subsystems/radio_control.h"