update pcspkdemo

c_os/user/demo/PCSPKdemo.cc

@@ -0,0 +1,8 @@
+#include "user/demo/PCSPKdemo.h"
+
+void PCSPKdemo::run() {
+    (pcspk.*this->melody)();  // This syntax is confusing as hell
+
+    pcspk.off();
+    scheduler.exit();
+}