scripts: apibuild: parse 'Since' for macros

scripts: apibuild: parse 'Since' for macros

This patch adds 'version' parameter to the generated XML API for
macros

It'll require, for new additions, to add a comment with the version
that the macro was added. An example bellow of code diff and
the change in the generated XML.

Note that the Since tag is removed from the comment as there is a
proper field for it in the XML.

Signed-off-by: Victor Toso <victortoso@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>