Add missing absl::optional includes

Bug: None
Change-Id: I4abece77b021a866175253cbb2bd212ff618910c
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/341022
Auto-Submit: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41813}
diff --git a/rtc_base/task_utils/repeating_task_unittest.cc b/rtc_base/task_utils/repeating_task_unittest.cc
index 2c269b4..de6cfdb 100644
--- a/rtc_base/task_utils/repeating_task_unittest.cc
+++ b/rtc_base/task_utils/repeating_task_unittest.cc
@@ -14,6 +14,7 @@
 #include <memory>
 
 #include "absl/functional/any_invocable.h"
+#include "absl/types/optional.h"
 #include "api/task_queue/task_queue_base.h"
 #include "api/task_queue/test/mock_task_queue_base.h"
 #include "api/units/time_delta.h"