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/src/core/SkPixmap.cpp b/src/core/SkPixmap.cpp
index f285668..57bb194 100644
--- a/src/core/SkPixmap.cpp
+++ b/src/core/SkPixmap.cpp
@@ -11,6 +11,7 @@
 #include "SkMask.h"
 #include "SkPixmap.h"
 #include "SkUtils.h"
+#include "SkPM4f.h"
 
 void SkAutoPixmapUnlock::reset(const SkPixmap& pm, void (*unlock)(void*), void* ctx) {
     SkASSERT(pm.addr() != nullptr);