Host Keepalive followup (#21290)

Followup to #21283

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

Marlin/src/HAL/ESP32/FlushableHardwareSerial.cpp

@@ -20,11 +20,10 @@
  *
  */
 
-#include "FlushableHardwareSerial.h"
-
 #ifdef ARDUINO_ARCH_ESP32
 
+#include "FlushableHardwareSerial.h"
 
 Serial0Type<FlushableHardwareSerial> flushableSerial(false, 0);
 
-#endif // ARDUINO_ARCH_ESP32
+#endif