obs-ffmpeg: Include avcodec header for AVCodecContext

FFmpeg commit e67e02d15672 [1] drops avcodec.h from avformat.h includes.
Therefore we need to include it explicitly.

[1]: e67e02d156
     lavf/avformat.h: drop the avcodec.h include

plugins/obs-ffmpeg/obs-ffmpeg-nvenc.c

@@ -24,6 +24,7 @@
 
 #include <libavutil/opt.h>
 #include <libavutil/pixdesc.h>
+#include <libavcodec/avcodec.h>
 #include <libavformat/avformat.h>
 
 #include "obs-ffmpeg-formats.h"