net/freeswitch: unbreak with ffmpeg 4.0

avcodec.c:194:40: error: use of undeclared identifier 'FF_INPUT_BUFFER_PADDING_SIZE'
static uint8_t ff_input_buffer_padding[FF_INPUT_BUFFER_PADDING_SIZE] = { 0 };

^

avformat.c:471:14: error: use of undeclared identifier 'CODEC_FLAG_LOOP_FILTER'

c->flags|=CODEC_FLAG_LOOP_FILTER;   // flags=+loop
          ^

avformat.c:477:47: error: use of undeclared identifier 'ME_HEX'

av_opt_set_int(c->priv_data, "motion_est", ME_HEX, 0);
                                           ^

avformat.c:553:15: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

c->flags |= CODEC_FLAG_GLOBAL_HEADER;
            ^

avformat.c:638:31: error: use of undeclared identifier 'CODEC_CAP_VARIABLE_FRAME_SIZE'

if (c->codec->capabilities & CODEC_CAP_VARIABLE_FRAME_SIZE) {
                             ^

PR: 227726
Reported by: antoine (via exp-run)