[rust png] Limit `libpng` visibility to `third_party/weston`.

This CL limits transitive `libpng` to specific targets underneath
`//third_party/weston`, and only exposes a single `//third_party/weston`
target (an `executable`) to `//ui/ozone/platform/wayland`.

Bug: 443128323
Change-Id: Idd40dde4d9a3df35e7bfedb0376820c2a7feb65f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6976915
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523851}
NOKEYCHECK=True
GitOrigin-RevId: 7936c66ac381ba218cfe87d7bc07872ea5020cea