graphics/libplacebo: always install headers after f23f6a9c6a74
https://code.videolan.org/videolan/libplacebo/-/commit/1956dbb57354
https://code.videolan.org/videolan/libplacebo/-/commit/9894cdf2ee27
https://code.videolan.org/videolan/libplacebo/-/commit/46494038d21d
PR: 269061
(cherry picked from commit 3199ade18ee5f63a8714929e01e073492e395d1e)