Add a missing SK_API to SkImageFilter::Cache.

include/core/SkImageFilter.h

Index: include/core/SkImageFilter.h
diff --git a/include/core/SkImageFilter.h b/include/core/SkImageFilter.h
index b3d748095571e3978cb63003197ff7be3af45d77..9cdd27599a8a09f1a5d681ed6efd723ffb823c25 100644
--- a/include/core/SkImageFilter.h
+++ b/include/core/SkImageFilter.h
@@ -48,7 +48,7 @@ public:
         uint32_t fFlags;
     };
 
-    class Cache : public SkRefCnt {
+    class SK_API Cache : public SkRefCnt {
     public:
         // By default, we cache only image filters with 2 or more children.
         static Cache* Create(int minChildren = 2);