An identical call exists in ffmpeg.c
With POSIX/glibc, stderr is already unbuffered (or line-buffered when
a terminal is connected), but not in case of MSVCRT.
Explicitly calling setvbuf() like in this commit, makes the Windows
runtime behave like POSIX, giving the same “print immediately” behavior.
Signed-off-by: softworkz <softworkz@hotmail.com>