rt: rust_sched_launcher needs a virtual destructor

Author: brson

src/rt/rust_sched_launcher.h

@@ -17,6 +17,7 @@ class rust_sched_launcher : public kernel_owned<rust_sched_launcher> {
 
 public:
     rust_sched_launcher(rust_scheduler *sched, rust_srv *srv, int id);
+    virtual ~rust_sched_launcher() { }
 
     virtual void start() = 0;
     virtual void join() = 0;