Fix build error

media_writer.cpp: In member function 'int media_writer::open(const string&, const stream_properties&)':
media_writer.cpp:281:50: error: invalid conversion from 'const AVOutputFormat*' to 'AVOutputFormat*' [-fpermissive]
  281 |         AVOutputFormat *fmt_out = av_guess_format("mp4", NULL, "video/mp4");
      |                                   ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                  |
      |                                                  const AVOutputFormat*

server/media_writer.cpp

@@ -278,7 +278,7 @@ int media_writer::open(const std::string &path, const stream_properties &propert
 	AVCodec *codec;
 
 	/* Get the output format */
-	AVOutputFormat *fmt_out = av_guess_format("mp4", NULL, "video/mp4");
+	const AVOutputFormat *fmt_out = av_guess_format("mp4", NULL, "video/mp4");
 	if (fmt_out == NULL)
 	{
 		bc_log(Error, "media_writer: MP4 output format is not found!");
@@ -707,4 +707,4 @@ int snapshot_writer::write_frame(AVFrame *rawFrame)
 
 ///////////////////////////////////////////////////////////////
 // S.K. >> END-OF: Implementation of separated snapshot writer
-///////////////////////////////////////////////////////////////
+///////////////////////////////////////////////////////////////