fix some declarations

src/modbus-rtu.c

@@ -317,8 +317,9 @@ static ssize_t _modbus_rtu_send(modbus_t *ctx, const uint8_t *req, int req_lengt
 
         return size;
     } else {
-#endif
+#else
     modbus_rtu_t *ctx_rtu = ctx->backend_data;
+#endif    
     ssize_t w;
     
     if(!ctx_rtu->echohw)