AVCodec is only ever used as an incomplete type (i.e. via a pointer
to an AVCodec) in avformat.h and it is not really part of the core
of avformat.h or libavformat; almost none of our internal users
make use of it (and none make use of hwcontext.h, which is implicitly
included). So switch to use struct AVCodec, but continue to include
codec.h for external users for compatibility.

Also, do the same for AVFrame and frame.h, which is implicitly included
by codec.h (via lavu/hwcontext.h).

Also, remove an unnecessary inclusion of <time.h>.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>