lavfi/avectorscope: use the name 's' for the pointer to the private context.
lavfi/sine: use the name 's' for the pointer to the private context.
lavfi/flite: use the name 's' for the pointer to the private context.
lavfi/aevalsrc: use the name 's' for the pointer to the private context.
lavfi/volumedetect: use the name 's' for the pointer to the private context.