Include <X11/Xutil.h> in gdkdnd-x11.c

Otherwise, inclusion of shape.h triggers an unknown type error
for Region, and compile fails, at least in Solaris.

Reported by Philip Brown here:
https://bugzilla.gnome.org/show_bug.cgi?id=615028

gdk/x11/gdkdnd-x11.c

@@ -25,7 +25,9 @@
  */
 
 #include "config.h"
+
 #include <X11/Xlib.h>
+#include <X11/Xutil.h>
 #include <X11/Xatom.h>
 #include <X11/extensions/shape.h>