Make DatastoreAccess::getItems const

I want to use this method in other const methods.

Change-Id: I32ff3fd8f0e77dbcea9b83b8ab8da99c1aa41f17
diff --git a/src/netconf_access.hpp b/src/netconf_access.hpp
index 3f45a27..924b2c2 100644
--- a/src/netconf_access.hpp
+++ b/src/netconf_access.hpp
@@ -33,7 +33,7 @@
     NetconfAccess(const std::string& socketPath);
     NetconfAccess(std::unique_ptr<libnetconf::client::Session>&& session);
     ~NetconfAccess() 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;