[skottie] Pull SkottieProperty.h from Skotti.h

Forward declaration for PropertyObserver should work, but some G3
builds/configs are barfing...

TBR=

Change-Id: I47fc8d24d4e706df470c010c8fce13f07d726fd8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293340
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/modules/skottie/include/Skottie.h b/modules/skottie/include/Skottie.h
index 7786909..c519068 100644
--- a/modules/skottie/include/Skottie.h
+++ b/modules/skottie/include/Skottie.h
@@ -14,6 +14,7 @@
 #include "include/core/SkString.h"
 #include "include/core/SkTypes.h"
 #include "modules/skottie/include/ExternalLayer.h"
+#include "modules/skottie/include/SkottieProperty.h"
 #include "modules/skresources/include/SkResources.h"
 
 #include <memory>
@@ -39,8 +40,6 @@
 using ImageAsset = skresources::ImageAsset;
 using ResourceProvider = skresources::ResourceProvider;
 
-class PropertyObserver;
-
 /**
  * A Logger subclass can be used to receive Animation::Builder parsing errors and warnings.
  */