DocBook: Do not exceed argument list limit

Use find + xargs to compress the generated manpages. Without this patch,
the build can fail with

    gzip -f Documentation/DocBook/man/*.9
    /bin/bash: /usr/bin/gzip: Argument list too long

This happened with qemu user mode emulation on aarch64.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>