lavfi: fix build on Libav

It doesn't provide this function. The code is not really designed to
work without it, so it will probably mess up big time, but at least
make it compile again.

player/lavfi.c

@@ -464,8 +464,10 @@ static void feed_input_pads(struct lavfi *c)
         pad->input_needed = false;
         pad->input_eof |= !pad->connected;
 
+#if LIBAVFILTER_VERSION_MICRO >= 100
         if (!av_buffersrc_get_nb_failed_requests(pad->buffer))
             continue;
+#endif
 
         if (c->draining_recover_eof || c->draining_new_format)
             continue;