clients/nested: fix undefined reference to xzalloc

Include shared/xalloc.h in clients/nested.c to fix
compilation error: undefined reference to `xzalloc'

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/nested.c

@@ -47,6 +47,7 @@
 #include <wayland-server.h>
 
 #include "shared/helpers.h"
+#include "shared/xalloc.h"
 #include "window.h"
 
 #ifndef EGL_WL_create_wayland_buffer_from_image