Fixes compilation problems with GCC 4.6.
From http://sourceforge.net/tracker/index.php?func=detail&aid=3308354&group_id=52551&atid=467232

--- a/include/dos_inc.h.old	2011-03-28 18:43:27.000000000 +0200
+++ b/include/dos_inc.h	2011-03-28 18:43:35.000000000 +0200
@@ -27,6 +27,7 @@
 #ifndef DOSBOX_MEM_H
 #include "mem.h"
 #endif
+#include <stddef.h>
 
 #ifdef _MSC_VER
 #pragma pack (1)