Move some includes around to uncover missing deps

Marlin/MarlinSerial.cpp

@@ -29,10 +29,12 @@
   Modified 14 February 2016 by Andreas Hardtung (added tx buffer)
 */
 
-#include "Marlin.h"
 #include "MarlinSerial.h"
+
 #include "stepper.h"
 
+#include "Marlin.h"
+
 #ifndef USBCON
 // this next line disables the entire HardwareSerial.cpp,
 // this is so I can support Attiny series and any other chip without a UART