[skottie/sksg] Move source files list to .gni

To facilitate Chromium integration.

Change-Id: Ie6a08e90feda4c6deace9754f0baef7176882334
Reviewed-on: https://skia-review.googlesource.com/145146
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed