[libpng] Add missing visibility item

Bug: 77808
Change-Id: Icb01d823825a3fd4d16d718f59f42527339e698b
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/537122
Reviewed-by: Shai Barack <shayba@google.com>
1 file changed