tools/xenpaging: Add _XOPEN_SOURCE to fix build problems with recent gcc

This patch fixes compilation issues with
gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21).

Signed-off-by: Daniel Kiper <dkiper@net-space.pl>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
xen-unstable changeset:   22023:af6799abc6e9
xen-unstable date:        Wed Aug 18 16:48:25 2010 +0100

tools/xenpaging/xenpaging.c

@@ -19,6 +19,7 @@
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  */
 
+#define _XOPEN_SOURCE	600
 
 #include <inttypes.h>
 #include <stdlib.h>