configure: Add (h264|hevc)_sei subsystems

Currently, several components select atsc_a53, despite
not using anything from it themselves. They only select
it because parsing SEI messages adds an indirect dependency.
But using direct dependencies is more natural, so add
dedicated subsystems for them.

It already allows to remove a superfluous dependency of
the HEVC QSV encoder on hevc_sei and atsc_a53.

Adding new subsystems only becomes effective after a reconfiguration.
In order to force this, some needed headers (which are only included
implicitly before this commit) were included explicitly in
libavformat/allformats.c.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

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