fftools/ffmpeg: stop including os_support.h

It was added for usleep(), which
- is not used in ffmpeg
- is not handled in os_support.h anymore

fftools/ffmpeg.c

@@ -63,7 +63,6 @@
 #include "libavutil/thread.h"
 #include "libavutil/threadmessage.h"
 #include "libavcodec/version.h"
-#include "libavformat/os_support.h"
 
 # include "libavfilter/avfilter.h"
 # include "libavfilter/buffersrc.h"