add missing include

need to explicitly include elapsedMillis due it being a separate package

src/SerEMU.cpp

@@ -22,6 +22,7 @@
  */
 
 #include <core/Arduino.h>
+#include <elapsedmillis/elapsedMillis.h>
 #include "usbhost/usbhost.h"  // Read this header first for key info
 
 //#define  SEREMU_PRINT_DEBUG