Make DatastoreAccess::getItems const

I want to use this method in other const methods.

Change-Id: I32ff3fd8f0e77dbcea9b83b8ab8da99c1aa41f17
diff --git a/src/sysrepo_access.cpp b/src/sysrepo_access.cpp
index 70b8823..e95d740 100644
--- a/src/sysrepo_access.cpp
+++ b/src/sysrepo_access.cpp
@@ -186,7 +186,7 @@
     }
 }
 
-DatastoreAccess::Tree SysrepoAccess::getItems(const std::string& path)
+DatastoreAccess::Tree SysrepoAccess::getItems(const std::string& path) const
 {
     using namespace std::string_literals;
     Tree res;
@@ -364,7 +364,7 @@
     return m_schema;
 }
 
-[[noreturn]] void SysrepoAccess::reportErrors()
+[[noreturn]] void SysrepoAccess::reportErrors() const
 {
     // I only use get_last_errors to get error info, since the error code from
     // sysrepo_exception doesn't really give any meaningful information. For