avfilter/formats: Constify channel_layout in ff_add_channel_layout()

It copies, not moves the channel layout.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavfilter/formats.c

@@ -464,7 +464,8 @@ do {                                                              \
     (*f)->nb++;                                                   \
 } while (0)
 
-int ff_add_channel_layout(AVFilterChannelLayouts **l, AVChannelLayout *channel_layout)
+int ff_add_channel_layout(AVFilterChannelLayouts **l,
+                          const AVChannelLayout *channel_layout)
 {
     av_assert1(!(*l && (*l)->all_layouts));
     ADD_FORMAT(l, channel_layout, ff_channel_layouts_unref, AVChannelLayout, channel_layouts, nb_channel_layouts);

libavfilter/formats.h

@@ -180,7 +180,8 @@ av_warn_unused_result
 int ff_set_common_formats_from_list(AVFilterContext *ctx, const int *fmts);
 
 av_warn_unused_result
-int ff_add_channel_layout(AVFilterChannelLayouts **l, AVChannelLayout *channel_layout);
+int ff_add_channel_layout(AVFilterChannelLayouts **l,
+                          const AVChannelLayout *channel_layout);
 
 /**
  * Add *ref as a new reference to f.