Don't compile STM32 SoftwareSerial unless STM32 (#15894)

Marlin/src/HAL/HAL_STM32/SoftwareSerial.cpp

 //
 // Includes
 //
+#if defined(ARDUINO_ARCH_STM32) && !defined(STM32GENERIC)
+
 #include "SoftwareSerial.h"
 #include "timers.h"
 
   // Read from "head"
   return _receive_buffer[_receive_buffer_head];
 }
+
+#endif // ARDUINO_ARCH_STM32 && !STM32GENERIC