accel: wrap unref in ifdef

This fixes the Debian stable build.

Change-Id: I5a59beb34e3379c8e950a4325ef62d3810be84f4

src/media/media_decoder.cpp

@@ -59,8 +59,10 @@ MediaDecoder::MediaDecoder() :
 
 MediaDecoder::~MediaDecoder()
 {
+#ifdef RING_ACCEL
     if (decoderCtx_->hw_device_ctx)
         av_buffer_unref(&decoderCtx_->hw_device_ctx);
+#endif
     if (decoderCtx_)
         avcodec_close(decoderCtx_);
     if (inputCtx_)