Make DatastoreAccess::getItems const

I want to use this method in other const methods.

Change-Id: I32ff3fd8f0e77dbcea9b83b8ab8da99c1aa41f17
diff --git a/src/yang_access.cpp b/src/yang_access.cpp
index 71aebf1..c509231 100644
--- a/src/yang_access.cpp
+++ b/src/yang_access.cpp
@@ -108,7 +108,7 @@
     m_datastore = lyWrap(datastore);
 }
 
-DatastoreAccess::Tree YangAccess::getItems(const std::string& path)
+DatastoreAccess::Tree YangAccess::getItems(const std::string& path) const
 {
     DatastoreAccess::Tree res;
     if (!m_datastore) {