This file has been exported since our minimum required version (0.7.0),
but it wasn't documented. Instead it was transitively included by
<jxl/decode.h> (but not jxl/encode.h), which ffmpeg relied on.

libjxl broke its API in libjxl/libjxl@66b959239355aef5255 by removing
the transitive include of version.h, and they do not plan on adding
it back. Instead they are choosing to leave the API backwards-
incompatible with downstream callers written for some fairly recent
versions of their API.

As a result, we include <jxl/version.h> to continue to build against
more recent versions of libjxl. The version macros removed are also
present in that file, so we no longer need to redefine them.
Signed-off-by: Leo Izen <leo.izen@gmail.com>