includes: ptrdiff_t is defined in stddef.h

ptrdiff_t is defined in the stddef.h header, so this
header must be included.

Do the #include in a common header to avoid doing it
all over the place.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

---
Patch not sent upstream, it's a no-longer-maintained branch.
To be noted, however, is that the current devel branch is
still missing that include.

diff -durN cegui06-0.6.2.orig/include/CEGUIBase.h cegui06-0.6.2/include/CEGUIBase.h
--- cegui06-0.6.2.orig//include/CEGUIBase.h	2008-02-11 15:38:27.000000000 +0100
+++ cegui06-0.6.2/include/CEGUIBase.h	2012-08-19 22:51:50.260385978 +0200
@@ -34,6 +34,7 @@
 #define _CEGUIBase_h_
 
 #include <cassert>
+#include <stddef.h>
 
 // bring in configuration options
 #include "CEGUIConfig.h"