doc/ffprobe.xsd: add build_date and build_time as optional attributes

This partly undoes 7b35a01.

The intent of patch 7b35a01 was to no longer use build_date and
build_time as attributes of programVersion, but the patch also had the
effect of making all records generated with an earlier ffprobe build with
build_date and build_time as invalid.

This patch puts the two attributes back but without mandating their use,
thus older ffprobe records as backwards compatible with the current
schema and their use is no longer required.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
