DocBook: Don't store mtime (or name) in compressed man pages

The mtime on a man page is the build time.  As gzip stores the mtime
and original name in the compressed file by default, this makes
compressed man pages unreproducible.  Neither of these are important
metadata in this case, so turn this off.

Reported-by: Jérémy Bobbio <lunar@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>