make SkPM4f private

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1713653002

Review URL: https://codereview.chromium.org/1713653002
diff --git a/include/core/SkXfermode.h b/include/core/SkXfermode.h
index 3b0f49d..94f49fa 100644
--- a/include/core/SkXfermode.h
+++ b/include/core/SkXfermode.h
@@ -18,6 +18,9 @@
 class GrXPFactory;
 class SkString;
 
+struct SkPM4f;
+typedef SkPM4f (*SkXfermodeProc4f)(const SkPM4f& src, const SkPM4f& dst);
+
 /** \class SkXfermode
  *
  *  SkXfermode is the base class for objects that are called to implement custom