[Open Screen] Fix compilation error in simulated_capturer.cc.

Fixes:

/standalone_sender/simulated_capturer.cc -o obj/cast/standalone_sender/cast_sender.simulated_capturer.o
../cast/standalone_sender/simulated_capturer.cc:49:29: error: no matching function for call to 'av_find_best_stream'
  const int stream_result = av_find_best_stream(format_context_.get(),
                            ^~~~~~~~~~~~~~~~~~~
/usr/include/x86_64-linux-gnu/libavformat/avformat.h:2297:5: note: candidate function not viable: no known conversion from 'const AVCodec **' to 'AVCodec **' for 5th argument
int av_find_best_stream(AVFormatContext *ic,
    ^
1 error generated.

Change-Id: Ifcecaf51089704005bcae51cc86bf80e4e004207
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/3573093
Reviewed-by: Ryan Keane <rwkeane@google.com>
Commit-Queue: Mark Foltz <mfoltz@chromium.org>
diff --git a/cast/standalone_sender/simulated_capturer.cc b/cast/standalone_sender/simulated_capturer.cc
index 74a3001..81f8a8a 100644
--- a/cast/standalone_sender/simulated_capturer.cc
+++ b/cast/standalone_sender/simulated_capturer.cc
@@ -45,7 +45,7 @@
     return;  // Capturer is halted (unable to start).
   }
 
-  const AVCodec* codec;
+  AVCodec* codec;
   const int stream_result = av_find_best_stream(format_context_.get(),
                                                 media_type_, -1, -1, &codec, 0);
   if (stream_result < 0) {