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

Before commit 2f76476549, 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>

libavfilter/formats.h

@@ -83,7 +83,7 @@ struct AVFilterFormats {
  *   channel count with unknown disposition with the same number of channels
  *   (e.g. AV_CH_LAYOUT_STEREO and FF_COUNT2LAYOUT(2).
  */
-typedef struct AVFilterChannelLayouts {
+struct AVFilterChannelLayouts {
     uint64_t *channel_layouts;  ///< list of channel layouts
     int    nb_channel_layouts;  ///< number of channel layouts
     char all_layouts;           ///< accept any known channel layout
@@ -91,7 +91,7 @@ typedef struct AVFilterChannelLayouts {
 
     unsigned refcount;          ///< number of references to this list
     struct AVFilterChannelLayouts ***refs; ///< references to this list
-} AVFilterChannelLayouts;
+};
 
 /**
  * Encode a channel count as a channel layout.