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

This works around some issues with Firefox's unified sources build system.

BUG=skia:5173
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1869613003

Review URL: https://codereview.chromium.org/1869613003

src/core/SkPM4fPriv.h

@@ -5,6 +5,9 @@
  * found in the LICENSE file.
  */
 
+#ifndef SkPM4fPriv_DEFINED
+#define SkPM4fPriv_DEFINED
+
 #include "SkColorPriv.h"
 #include "SkPM4f.h"
 
@@ -56,3 +59,5 @@ static inline uint32_t Sk4f_toL32(const Sk4f& x4) {
 static inline uint32_t Sk4f_toS32(const Sk4f& x4) {
     return to_4b(linear_to_srgb(x4) * Sk4f(255) + Sk4f(0.5f));
 }
+
+#endif