Rename gni group to be libpng specific

We have both libpng and rust png implementations.

Bug: b/381900683
Change-Id: I9f107f3e6f9eb722d9ee50ce536da1521b147284
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1139996
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Łukasz Anforowicz <lukasza@google.com>
Reviewed-by: Łukasz Anforowicz <lukasza@google.com>
3 files changed