package/libplacebo: add optional dependencies to lcms & xxhash

Buildroot commit 006aab8d64d2179d77df9a696b3fd8636f541640 added the
package at version v7.351.0.

This version includes optional upstream support for

lcms, added in v0.5.0:
https://code.videolan.org/videolan/libplacebo/-/commit/eb14d3bdf22605263a9b6025ed6280eb1515a89a

xxhash, added in v6.338.0:
https://code.videolan.org/videolan/libplacebo/-/commit/cb1009d1455e64787e52c5c3b6f7042aa37e5c14

This patch adds handling of these optional dependencies.

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Julien Olivain <ju.o@free.fr>