FREI0R path broken in libmlt7 v7.12.0-dmo2

The most recent deb-multimedia versions of libmlt7 and kdenlive in
bookworm are broken - the frei0r plugins are no longer being detected.

I presume you used to have a patch to modules/frei0r/factory.c to add
usr/lib/x86_64-linux-gnu/frei0r-1 to FREI0R_PLUGIN_PATH or something.

For more info see https://github.com/mltframework/mlt/issues/855