Marlin.pde: include comment for emacs mode

Emacs by default doesn't recognise a ".pde" file as C++ source code.
Add the annotation to the top of the file to make it work.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

Marlin/Marlin.pde

+/* -*- c++ -*- */
+
 /*
     Reprap firmware based on Sprinter and grbl.
  Copyright (C) 2011 Camiel Gubbels / Erik van der Zalm