Fix Host Keepalive serial target (#21283)

Co-authored-by: Scott Lahteine <github@thinkyhead.com>

Marlin/src/HAL/ESP32/FlushableHardwareSerial.h

@@ -21,9 +21,9 @@
  */
 #pragma once
 
-#ifdef ARDUINO_ARCH_ESP32
-
 #include <HardwareSerial.h>
+
+#include "../shared/Marduino.h"
 #include "../../core/serial_hook.h"
 
 class FlushableHardwareSerial : public HardwareSerial {
@@ -32,5 +32,3 @@ public:
 };
 
 extern Serial0Type<FlushableHardwareSerial> flushableSerial;
-
-#endif // ARDUINO_ARCH_ESP32