Fix builds for which UART_TX_LED aren't defined

src/cdc_uart.c

diff --git a/src/cdc_uart.c b/src/cdc_uart.c
index c6e4c67..6f2450a 100644
--- a/src/cdc_uart.c
+++ b/src/cdc_uart.c
@@ -146,7 +146,9 @@
         if (((int)break_expiry - (int)xTaskGetTickCount()) < 0) {
           timed_break = false;
           uart_set_break(PROBE_UART_INTERFACE, false);
+#ifdef PROBE_UART_TX_LED
           tx_led_debounce = 0;
+#endif
         } else {
           keep_alive = true;
         }
@@ -156,7 +158,9 @@
       uart_set_break(PROBE_UART_INTERFACE, false);
       timed_break = false;
       was_connected = 0;
+#ifdef PROBE_UART_TX_LED
       tx_led_debounce = 0;
+#endif
       cdc_tx_oe = 0;
     }
     return keep_alive;
@@ -278,15 +282,19 @@
     case 0xffff:
     uart_set_break(PROBE_UART_INTERFACE, true);
     timed_break = false;
+#ifdef PROBE_UART_TX_LED
     gpio_put(PROBE_UART_TX_LED, 1);
     tx_led_debounce = 1 << 30;
+#endif
     break;
     default:
     uart_set_break(PROBE_UART_INTERFACE, true);
     timed_break = true;
+#ifdef PROBE_UART_TX_LED
     gpio_put(PROBE_UART_TX_LED, 1);
-    break_expiry = xTaskGetTickCount() + (wValue * (configTICK_RATE_HZ / 1000));
     tx_led_debounce = 1 << 30;
+#endif
+    break_expiry = xTaskGetTickCount() + (wValue * (configTICK_RATE_HZ / 1000));
     break;
   }
 }