Add #pragma once to cereal_optional.hpp

Reviewers: mferencevic

Reviewed By: mferencevic

Differential Revision: https://phabricator.memgraph.io/D1080

src/utils/cereal_optional.hpp

@@ -1,3 +1,5 @@
+#pragma once
+
 #include <experimental/optional>
 
 namespace cereal {