[third_party] Restrict libpng visibility to existing users Fix typo Bug: 77808 Change-Id: Ic6314bf1f37ad98169c54a7d218f570273c7d9e1 Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/537121 Reviewed-by: Shai Barack <shayba@google.com>