Fix transitive dependencies on SkImageEncoder

In https://skia-review.googlesource.com/c/skia/+/512416,
we would like to decouple SkImage and SkImageEncoder. This CL
was created by searching for use of these objects:
 - SkEncodedImageFormat
 - SkStream
 - SkData
 - SkBitmap
 - SkPixmap

and making sure those files followed the Include What You Use
(IWYU) guidelines.

Signed-off-by: Kevin Lubick <kjlubick@google.com>
Change-Id: I8edbcd1c9a526b8084d7e2c023895d1ad2f8c9b1
diff --git a/libs/hwui/Readback.h b/libs/hwui/Readback.h
index d0d748f..aa6e43c 100644
--- a/libs/hwui/Readback.h
+++ b/libs/hwui/Readback.h
@@ -20,7 +20,11 @@
 #include "Rect.h"
 #include "renderthread/RenderThread.h"
 
-#include <SkBitmap.h>
+#include <SkRefCnt.h>
+
+class SkBitmap;
+class SkImage;
+struct SkRect;
 
 namespace android {
 class Bitmap;