Make DatastoreAccess::getItems const

I want to use this method in other const methods.

Change-Id: I32ff3fd8f0e77dbcea9b83b8ab8da99c1aa41f17
diff --git a/src/netconf_access.cpp b/src/netconf_access.cpp
index bcf6fb4..dccec1c 100644
--- a/src/netconf_access.cpp
+++ b/src/netconf_access.cpp
@@ -16,7 +16,7 @@
 
 NetconfAccess::~NetconfAccess() = default;
 
-DatastoreAccess::Tree NetconfAccess::getItems(const std::string& path)
+DatastoreAccess::Tree NetconfAccess::getItems(const std::string& path) const
 {
     Tree res;
     auto config = m_session->get((path != "/") ? std::optional{path} : std::nullopt);