vf: Fix filter enabling logic for vf_uspp and vfs using lavc internals

vf: Fix filter enabling logic for vf_uspp and vfs using lavc internals

Move vf_uspp from the list of filters depending on internal non-API
FFmpeg code to the list of filters only depending on libavcodec
normally - at least it doesn't need any internal headers, and I don't
see other obvious unsafe non-API usage either.

Change a leftover CONFIG_LIBAVCODEC_A test to the now correct
CONFIG_LIBAVCODEC_INTERNALS to actually allow using the video filters
requiring non-API libavcodec internals when --ffmpeg-source-dir was
specified.