add header guard to SkPM4fPriv.h to prevent accidental multiple inclusion

src/core/SkPM4fPriv.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
+#ifndef SkPM4fPriv_DEFINED
+#define SkPM4fPriv_DEFINED
+
 #include "SkColorPriv.h"
 #include "SkPM4f.h"
 
 static inline float get_alpha(const Sk4f& f4) {
     return f4[SkPM4f::A];
 }
 
 static inline Sk4f set_alpha(const Sk4f& f4, float alpha) {
     static_assert(3 == SkPM4f::A, "");
     return Sk4f(f4[0], f4[1], f4[2], alpha);
@@ skipping 31 matching lines @@
     return srgb_to_linear(to_4f(src) * Sk4f(1.0f/255));
 }
 
 static inline uint32_t Sk4f_toL32(const Sk4f& x4) {
     return to_4b(x4 * Sk4f(255) + Sk4f(0.5f));
 }
 
 static inline uint32_t Sk4f_toS32(const Sk4f& x4) {
     return to_4b(linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
 }
+
+#endif