package/luksmeta: do not build man pages

package/luksmeta: do not build man pages

Fixes:
http://autobuild.buildroot.net/results/a6247b95f1578fe1daec485589582310c75b5d84/

luksmeta-v9 generates man pages at build if a2x is available since:

commit 3fa51bb22350fee101fc52044949f6eb394114ae
Author: Daniel Kopeček <dkopecek@redhat.com>
Date:   Fri Jul 13 01:52:45 2018 +0200

   Generate manual page from source during build time

   If a2x (asciidoc) is not available during configure time,
   a warning will be generated and the manual page wont be
   generated nor installed.

Man pages are not needed on target and the build step fails in certain
setups, so disable it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>