multimedia/jellyfin-ffmpeg7: don't advertise private libraries

The bundled libs install into a private prefix and are loaded by
ffmpeg/ffprobe through --enable-rpath, so nothing else links against them.

Drop USE_LDCONFIG and trim INSTALL_TARGET (no install-headers) to match
upstream's packaging.

Only for archs amd64, aarch64 -- upstream targets 64bit

Suggested by: arrowd