Can't put a flexible array member in a union... add another anonymous struct layer.

src/sftp/sftp.h

@@ -69,8 +69,10 @@ typedef struct sftp_tx_pkt {
 			uint8_t type;
 			uint8_t data[];
 		};
+		struct {
 			uint8_t tdata[];
 		};
+	};
 } *sftp_tx_pkt_t;
 
 typedef struct sftp_rx_pkt {