obs-ffmpeg: Further FFmpeg deprecations fixes for FFmpeg 4.4+

avcodec.h stopped including channel_layout.h per FFmpeg commit
1be3d8a0cb77 [1]. Fixes compilation error on macOS against
FFmpeg later than the mentioned commit.
[1] https://github.com/FFmpeg/FFmpeg/commit/1be3d8a0cb77f8d34c1f39b47bf5328fe10c82d7

plugins/obs-ffmpeg/ffmpeg-mux/ffmpeg-mux.c

@@ -29,6 +29,7 @@
 #include <util/dstr.h>
 #include <libavcodec/avcodec.h>
 #include <libavformat/avformat.h>
+#include <libavutil/channel_layout.h>
 
 #define ANSI_COLOR_RED "\x1b[0;91m"
 #define ANSI_COLOR_MAGENTA "\x1b[0;95m"

plugins/obs-ffmpeg/obs-ffmpeg-output.c

@@ -25,6 +25,7 @@
 #include "obs-ffmpeg-output.h"
 #include "obs-ffmpeg-formats.h"
 #include "obs-ffmpeg-compat.h"
+#include <libavutil/channel_layout.h>
 
 struct ffmpeg_output {
 	obs_output_t *output;