avfilter: Add av_cold attributes to init/uninit functions

libavfilter/af_amix.c

libavfilter/af_ashowinfo.c

libavfilter/af_asyncts.c

libavfilter/af_channelsplit.c

libavfilter/af_join.c

libavfilter/af_volume.c

libavfilter/split.c

libavfilter/vf_hqdn3d.c

libavfilter/vf_lut.c

libavfilter/vsrc_movie.c

libavfilter/x86/af_volume_init.c