api(deltachat-ffi): make WebXdcRealtimeData data usable in CFFI

Previously only msg_id was returned to CFFI
without any way to get to the actual received data.

deltachat-ffi/deltachat.h

@@ -6284,6 +6284,18 @@ void dc_event_unref(dc_event_t* event);
 
 #define DC_EVENT_WEBXDC_INSTANCE_DELETED          2121
 
+/**
+ * Data received over an ephemeral peer channel.
+ *
+ * @param data1 (int) msg_id
+ * @param data2 (int) + (char*) binary data.
+ *     length is returned as integer with dc_event_get_data2_int()
+ *     and binary data is returned as dc_event_get_data2_str().
+ *     Binary data must be passed to dc_str_unref() afterwards.
+ */
+
+#define DC_EVENT_WEBXDC_REALTIME_DATA             2150
+
 /**
  * Tells that the Background fetch was completed (or timed out).
  *