Include SkRect in rtree. (#42050)

fixes flutter/flutter#126864

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: gaaclarke

flow/rtree.cc

@@ -8,6 +8,7 @@
 
 #include "flutter/fml/logging.h"
 #include "third_party/skia/include/core/SkBBHFactory.h"
+#include "third_party/skia/include/core/SkRect.h"
 
 namespace flutter {