docs-xml: Sort input file list

docs-xml: Sort input file list

because filesystems return entries in undeterministic order
and that ends up in index.xml and influences index.html
preventing reproducible builds of samba packages (e.g. for openSUSE)

See https://reproducible-builds.org/ for why this matters

Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Autobuild-User(master): David Disseldorp <ddiss@samba.org>
Autobuild-Date(master): Tue Jun 27 16:56:06 CEST 2017 on sn-devel-144