[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>
diff --git a/BUILD.gn b/BUILD.gn
index 27886f3..cb6b76d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -80,5 +80,6 @@
     "//src/ui/tools/gltf_export/*",
     "//src/ui/examples/frame-compression/*",
     "//src/developer/forensics/feedback_data/*",
+    "//third_party/freetype2/*",
   ]
 }