[FBcode->GH] Fix using namespace in pytorch/vision/torchvision/csrc/io/video/video… (#8481)

Co-authored-by: Richard Barnes <rbarnes@meta.com>

torchvision/csrc/io/video/video.h

@@ -6,8 +6,6 @@
 #include "../decoder/memory_buffer.h"
 #include "../decoder/sync_decoder.h"
 
-using namespace ffmpeg;
-
 namespace vision {
 namespace video {