removing const to make compatible with newer ffmpeg

libalmond/src/VideoStream.cpp

@@ -27,7 +27,7 @@ VideoStream::VideoStream(int width, int height, const std::string& filename, int
         throw VideoExportException{ "could not find h264 encoder" };
     }
 
-    const AVOutputFormat* oformat = av_guess_format(nullptr, filename.c_str(), nullptr);
+    AVOutputFormat* oformat = av_guess_format(nullptr, filename.c_str(), nullptr);
     if (!oformat)
         oformat = av_guess_format("mp4", nullptr, nullptr);
     if (oformat == nullptr)