Added destructor to CMSIS_RTOS2 mutex

include/etl/mutex/mutex_cmsis_os2.h

@@ -44,12 +44,17 @@ namespace etl
   public:
 
     mutex()
-      : id(0)
+      : id(NULL)
     {
       osMutexAttr_t attr = { "ETL", osMutexRecursive | osMutexPrioInherit | osMutexRobust, 0, 0 };
       id = osMutexNew(&attr);
     }
 
+    ~mutex()
+    {
+      osMutexDelete(id);
+    }
+
     void lock()
     {
       osMutexAcquire(id, osWaitForever);