icecast: Do not use chunked post

Icecast uses HTTP 1.0 while Libav uses HTTP 1.1 and enables by
default chunked post.

Icecast actually forwards the HTTP chunk headers to the listener
as part of the media stream (without the chunk encoding HTTP headers)
causing the players to lose sync.

Disabling the option is enough to feed icecast properly.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>