Fix ffmpeg detection with -D OPENCV_WARNINGS_ARE_ERRORS=ON option.

cmake/checks/ffmpeg_test.cpp

@@ -15,12 +15,15 @@ static void test()
   AVFormatContext* c = 0;
   AVCodec* avcodec = 0;
   AVFrame* frame = 0;
+  (void)avcodec;
+  (void)frame;
 
 #if LIBAVFORMAT_BUILD >= CALC_FFMPEG_VERSION(52, 111, 0)
   int err = avformat_open_input(&c, "", NULL, NULL);
 #else
   int err = av_open_input_file(&c, "", NULL, 0, NULL);
 #endif
+  (void)err;
 }
 
 int main() { test(); return 0; }