Tests/X11: Add missing include <stdlib.h> for 'rand'

Tests/X11/HelloWorldX11.cxx

@@ -15,6 +15,7 @@
 #define MAIN_H 1
 
 #include <iostream>
+#include <stdlib.h>
 
 /* include the X library headers */
 #include <X11/Xlib.h>