Fix minor compile error due to uninitialized field

main/app_serial.cpp

@@ -24,6 +24,7 @@ QueueHandle_t setupSerial(const uart_port_t uart)
     .parity = UART_PARITY_DISABLE,
     .stop_bits = UART_STOP_BITS_1,
     .flow_ctrl = UART_HW_FLOWCTRL_DISABLE,
+    .rx_flow_ctrl_thresh = UART_HW_FLOWCTRL_DISABLE,
     .source_clk = UART_SCLK_DEFAULT,
   };