Cast NULL to uintptr_t when calling rust_task::start().

On gcc 4.5.1 (fedora 14 i686) rust_test_runtime.cpp fails to compile
with the following error:

  rt/test/rust_test_runtime.cpp:57:61: error: passing NULL to non-pointer argument 3 of ‘void rust_task::start(uintptr_t, uintptr_t, uintptr_t, size_t)’

The explicit cast works around the error.

src/rt/test/rust_test_runtime.cpp

@@ -54,7 +54,7 @@ rust_task_test::worker::run() {
         kernel->create_domain(crate, "test");
     rust_dom *domain = handle->referent();
     domain->root_task->start(crate->get_exit_task_glue(),
-                             (uintptr_t)&task_entry, NULL, 0);
+                             (uintptr_t)&task_entry, (uintptr_t)NULL, 0);
     domain->start_main_loop();
     kernel->destroy_domain(domain);
 }