The png.h header provides some macros and declarations from subincludes.
By default include-what-you-use will suggest using these subincludes
directly, but this was not the intent of png.h. This adds a mapping file
so that include-what-you-use knows that these subincludes are actually
private and png.h should be used instead.
Change-Id: Ibf9df6dbdbde0e657f6e548ed0e8f2310f44cfea
Reviewed-on: https://skia-review.googlesource.com/114481
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
