Fix HAL_STM32 + Arduino IDE SoftwareSerial conflict (#16266)

Marlin/src/HAL/HAL_STM32/SoftwareSerial.cpp

@@ -34,7 +34,7 @@
 //
 // Includes
 //
-#if defined(ARDUINO_ARCH_STM32) && !defined(STM32GENERIC)
+#if defined(PLATFORMIO) && defined(ARDUINO_ARCH_STM32) && !defined(STM32GENERIC)
 
 #include "SoftwareSerial.h"
 #include "timers.h"