DocBook: Make mandocs parallel-safe

Two concurrent calls to cmd_db2man may attempt to compress manual
pages generated by each other.  gzip can then fail due to an input
file having already been compressed and removed.

Move the gzip command to the top-level mandocs target.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Bastian Blank <waldi@debian.org>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>