stdarg.h has been included in 780d7897
for ff_url_join(). This header became unnecessary when this function was
moved into a separate file in df9f22d4.

libavutil/pixdesc.h has been included for av_get_pix_fmt_name() in
603b8bc2 and is unused since commit
2fb75019 that removed the stuff belonging
to FF_API_FORMAT_PARAMETERS. Notice that this file still uses
AV_PIX_FMT_NONE and that therefore the header libavutil/pixfmt.h has
been included (this header is included in pixdesc.h as well as also in
libavutil/internal.h which is also included).

libavutil/time_internal.h has been included for gmtime_r() in commit
e7dd97b5; it is unused since commit
b72a7b96 which basically moved the code
making use of gmtime_r() to libavutil/dict.c to use in
avpriv_dict_set_timestamp().

audiointerleave.h has been added in c26e58e3
because of ff_interleave_compare_dts() (at that time the muxing code
was not split from utils.c yet); said function became static in commit
101e1f6f, making this header redundant.

metadata.h has been mostly included for what now resides in
libavutil/dict.h. The stuff that now resides in metadata.h has only been
used briefly: From commits ed7694d8 to
d60a9f52.

riff.h has been added in 45da8124
because riff.h once contained declarations for (ff_)codec_get_tag().
This was changed in bfe5454c.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>