avfilter/formats: Don't typedef structure a second time

avfilter/formats: Don't typedef structure a second time

Before commit 2f76476549a01ae2c4ec2a440e4b14c5aba64869, avfilter.h
contained no typedef for AVFilterChannelLayouts; all references to it
were done using its struct tag. formats.h meanwhile contained the
definition of the struct and a typedef for it. Said commit now added a
typedef in avfilter.h, too, bringing it in line with AVFilterFormats;
yet this means that there are two typedefs for AVFilterChannelLayouts
(in contrast to AVFilterFormats which is only typedef'ed in avfilter.h).

The problem is that older versions of GCC don't like this and error out:
http://fate.ffmpeg.org/history.cgi?slot=x86_64-openbsd5.6-gcc4.2-conf2
is one of the FATE boxes that now fail to compile. So just remove the
typedef in formats.h.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>