Make DatastoreAccess::getItems const

I want to use this method in other const methods.

Change-Id: I32ff3fd8f0e77dbcea9b83b8ab8da99c1aa41f17
diff --git a/src/yang_access.hpp b/src/yang_access.hpp
index 9f20b51..9b8a443 100644
--- a/src/yang_access.hpp
+++ b/src/yang_access.hpp
@@ -22,7 +22,7 @@
 public:
     YangAccess();
     ~YangAccess() override;
-    Tree getItems(const std::string& path) override;
+    Tree getItems(const std::string& path) const override;
     void setLeaf(const std::string& path, leaf_data_ value) override;
     void createItem(const std::string& path) override;
     void deleteItem(const std::string& path) override;