[rust png] Add @fmalita and @lukasza to `third_party/libpng/OWNERS`.

Bug: 443128323
Change-Id: I3c64d89d307873fb9f218ae67b7ec7703c74e68b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7003861
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: Chris Blume <cblume@chromium.org>
Reviewed-by: Chris Blume <cblume@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523849}
NOKEYCHECK=True
GitOrigin-RevId: c9c83060f0335327cb0666c95c42bc6def0b913c