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>