[third_party] Restrict libpng visibility to existing users

It's old code from 2016 and should probably be refreshed before we allow
it to be used in more places.

Bug: 77808
Change-Id: I09747f4adff38eb13f356911f6d68445fcc6a660
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/537021
Reviewed-by: Bryan Henry <bryanhenry@google.com>
1 file changed