[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>
diff --git a/BUILD.gn b/BUILD.gn
index 353b600..27886f3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -75,7 +75,7 @@
 
   # TODO(fxbug.dev/77808): pull new code for libpng before we allow it to be
   # used in more places.
-  visiblity = [
+  visibility = [
     "//src/camera/bin/factory/*",
     "//src/ui/tools/gltf_export/*",
     "//src/ui/examples/frame-compression/*",