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/Float16Test.cpp b/tests/Float16Test.cpp
index 3b13533..ef349c6 100644
--- a/tests/Float16Test.cpp
+++ b/tests/Float16Test.cpp
@@ -10,6 +10,7 @@
 #include "SkHalf.h"
 #include "SkOpts.h"
 #include "SkPixmap.h"
+#include "SkPM4f.h"
 #include "SkRandom.h"
 
 static bool eq_within_half_float(float a, float b) {