Binder: Flattenable is no longer a hard dependency

Bug: 302723053
Test: mma
Change-Id: I0dddbd085b1999c3eb11577e6120ea422be4c10a
diff --git a/libs/binder/tests/binderSafeInterfaceTest.cpp b/libs/binder/tests/binderSafeInterfaceTest.cpp
index 1c13866..cbbbe74 100644
--- a/libs/binder/tests/binderSafeInterfaceTest.cpp
+++ b/libs/binder/tests/binderSafeInterfaceTest.cpp
@@ -28,6 +28,7 @@
 #include <gtest/gtest.h>
 #pragma clang diagnostic pop
 
+#include <utils/Flattenable.h>
 #include <utils/LightRefBase.h>
 #include <utils/NativeHandle.h>