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