Add include to make g3 happy (#38850)

diff --git a/impeller/scene/animation/animation_clip.h b/impeller/scene/animation/animation_clip.h
index 8fe150e..60f5d17 100644
--- a/impeller/scene/animation/animation_clip.h
+++ b/impeller/scene/animation/animation_clip.h
@@ -5,6 +5,7 @@
 #pragma once
 
 #include <memory>
+#include <unordered_map>
 #include <vector>
 
 #include "flutter/fml/macros.h"