With 33d18982, the commit introducing
the new packet-based bsf API, a new buffer was no longer allocated
directly, but via av_new_packet(), so that libavutil/mem.h was no longer
needed.

Moreover since commit dc99ee6b
av_packet_make_writable() is employed which copies the data in case it
is unavoidable so that string.h is no longer used (it was used for
memcpy()).
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>