Add missing #include

Change-Id: I437664b4c1ff2a221093377bfd2a76cab13d5aed
diff --git a/aos/events/simulated_event_loop.h b/aos/events/simulated_event_loop.h
index 7718cfb..c8029de 100644
--- a/aos/events/simulated_event_loop.h
+++ b/aos/events/simulated_event_loop.h
@@ -2,6 +2,7 @@
 #define AOS_EVENTS_SIMULATED_EVENT_LOOP_H_
 
 #include <algorithm>
+#include <functional>
 #include <map>
 #include <memory>
 #include <string_view>