Include what you use, and you use std::optional

Change-Id: I212c5c7b085aa8cc646639fd4bcf72a66577b819
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/177035
Tested-by: Trond Norbye <trond.norbye@couchbase.com>
Reviewed-by: Richard de Mellow <richard.demellow@couchbase.com>

utilities/string_utilities.h

@@ -12,6 +12,7 @@
 #pragma once
 
 #include <platform/sized_buffer.h>
+#include <optional>
 #include <ostream>
 #include <string>
 #include <unordered_map>