Fix CI for Mac OS

[dforsi]

This PR tries to work around missing a build failure in the CI for Mac:

/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:282:46: error: no member named 'channels' in 'AVCodecContext'
    m_metaData.Channels = m_videoDecoderCtx->channels;
                          ~~~~~~~~~~~~~~~~~  ^
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:308:48: error: no member named 'channels' in 'AVCodecContext'
        << " channels: " << m_audioDecoderCtx->channels
                            ~~~~~~~~~~~~~~~~~  ^
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:309:54: error: no member named 'channel_layout' in 'AVCodecContext'
        << " channel_layout: " << m_audioDecoderCtx->channel_layout
                                  ~~~~~~~~~~~~~~~~~  ^
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:628:72: error: no member named 'channels' in 'AVCodecContext'
    av_opt_set_int(m_audioSWR, "in_channel_count",  m_audioDecoderCtx->channels, 0);
                                                    ~~~~~~~~~~~~~~~~~  ^
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:630:73: error: no member named 'channel_layout' in 'AVCodecContext'
    av_opt_set_int(m_audioSWR, "in_channel_layout",  m_audioDecoderCtx->channel_layout, 0);
                                                     ~~~~~~~~~~~~~~~~~  ^
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:640:57: error: no member named 'channels' in 'AVCodecContext'
        << " in_channel_count: " <<  m_audioDecoderCtx->channels
                                     ~~~~~~~~~~~~~~~~~  ^
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:642:58: error: no member named 'channel_layout' in 'AVCodecContext'
        << " in_channel_layout: " <<  m_audioDecoderCtx->channel_layout
                                      ~~~~~~~~~~~~~~~~~  ^
7 errors generated.```

An alternative that could be tested is disabling the plugins that use libavcodec
-DENABLE_CHANNELRX_DEMODDATV=OFF
-DENABLE_CHANNELTX_MODDATV=OFF