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/tests/SkLinearBitmapPipelineTest.cpp b/tests/SkLinearBitmapPipelineTest.cpp
index db1fc2a..851b201 100644
--- a/tests/SkLinearBitmapPipelineTest.cpp
+++ b/tests/SkLinearBitmapPipelineTest.cpp
@@ -4,10 +4,10 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
+
 #include "SkLinearBitmapPipeline.h"
-
 #include "SkColor.h"
-
+#include "SkPM4f.h"
 #include "Test.h"
 
 struct SinkBilerpProcessor final : public PointProcessorInterface {