Use updated gni source list

[kjlubick]

In https://skia-review.googlesource.com/c/skia/+/884096, Skia broke up this gni file list into Ganesh and Graphite specific code. Since Flutter only needs Ganesh sources, this can be made more precise.

Pre-launch Checklist

• [ x ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ x ] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [ x ] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• [ x ] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [ x ] I updated/added relevant documentation (doc comments with ///).
• [ x ] I signed the CLA.
• [ x ] All existing and new tests are passing.

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[flar]

RSLGTM