Add missing libc++ includes

Bug: 175635923
Test: m checkbuild
Change-Id: I9cef8d1b0306fab363e2fe4eda39782e21f24d8f
diff --git a/services/surfaceflinger/common/include/common/FlagManager.h b/services/surfaceflinger/common/include/common/FlagManager.h
index 2f2895c..c01465b 100644
--- a/services/surfaceflinger/common/include/common/FlagManager.h
+++ b/services/surfaceflinger/common/include/common/FlagManager.h
@@ -17,6 +17,7 @@
 #pragma once
 
 #include <cstdint>
+#include <functional>
 #include <mutex>
 #include <optional>
 #include <string>