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
1 file changed