schema: improve List::keys() not to use std::move

`List::keys()` used `std::move` while iterating over array of leafs.
This was solved without using `std::move`.

Change-Id: I8cbf8780ecd8848e46c1de5d4123a08624536bba

Author: bedrich-schindler

src/SchemaNode.cpp

@@ -593,8 +593,7 @@ std::vector<Leaf> List::keys() const
     LY_LIST_FOR(list->child, elem)
     {
         if (lysc_is_key(elem)) {
-            Leaf leaf(elem, m_ctx);
-            res.emplace_back(std::move(leaf));
+            res.emplace_back(Leaf(elem, m_ctx));
         }
     }