Make variables that are private in ConditionVariable.h, protected

Paritally reverts docs PR 8511

Author: Cruz Monrreal II

rtos/ConditionVariable.h

@@ -318,7 +318,6 @@ class ConditionVariable : private mbed::NonCopyable<ConditionVariable> {
         bool in_list;
     };
 
-private:
     static void _add_wait_list(Waiter **wait_list, Waiter *waiter);
     static void _remove_wait_list(Waiter **wait_list, Waiter *waiter);
     Mutex &_mutex;