Guards around usage of std::initializer_list in optional.h

In optional.h, ETL_HAS_INITIALIZER_LIST is being used to
guard against cases where std::initializer_list is not available.
But not consistently. This changes fixes it by adding it in the
remaining places.

Author: rolandreichweinbmw

include/etl/optional.h

@@ -195,6 +195,7 @@ namespace etl
         storage.construct(etl::forward<TArgs>(args)...);
       }
 
+#if ETL_HAS_INITIALIZER_LIST
       //*******************************************
       /// Construct from initializer_list and arguments.
       //*******************************************
@@ -205,6 +206,7 @@ namespace etl
       {
         storage.construct(ilist, etl::forward<TArgs>(args)...);
       }
+#endif
 #endif
 
       //***************************************************************************
@@ -816,6 +818,7 @@ namespace etl
         storage.construct(etl::forward<TArgs>(args)...);
       }
 
+#if ETL_HAS_INITIALIZER_LIST
       //*******************************************
       /// Construct from initializer_list and arguments.
       //*******************************************
@@ -826,6 +829,7 @@ namespace etl
       {
         storage.construct(ilist, etl::forward<TArgs>(args)...);
       }
+#endif
 #endif
 
       //***************************************************************************