Fix heap initialization error in rtos-less code

Author: ccli8

platform/mbed_retarget.cpp

@@ -42,6 +42,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include "platform/mbed_retarget.h"
+#include "mbed_rtx.h"
 
 static SingletonPtr<PlatformMutex> _mutex;