pioarduino Heltec v4: fix build due to LED_BUILTIN compile error. (#9875)

Fixes this build error:

  <command-line>: error: expected unqualified-id before '-' token
  /home/<snip>/.platformio/packages/framework-arduinoespressif32/variants/esp32s3/pins_arduino.h:15:22: note: in expansion of macro 'LED_BUILTIN'
     15 | static const uint8_t LED_BUILTIN = SOC_GPIO_PIN_COUNT + PIN_RGB_LED;
        |                      ^~~~~~~~~~~

More info: #9122 (comment)

The fix is consistent with variants/esp32s3/heltec_v3/platformio.ini

This commit is intentionally on the develop branch, because it's harmless to
develop branch, and makes us more ready for pioarduino when the time comes.

Heltec v4 introduced in: #7845

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

Author: cpatulea

variants/esp32s3/heltec_v4/platformio.ini

@@ -8,6 +8,7 @@ build_flags =
   -D HELTEC_V4
   -D HAS_LORA_FEM=1
   -I variants/esp32s3/heltec_v4
+  -ULED_BUILTIN
 
 
 [env:heltec-v4]