package/openldap: do not build the doc

Building the documentation fails on some autobuilders,
so disable it as we're not interested in it.

Fixes:
    http://autobuild.buildroot.org/results/1dc/1dc17a54e57130bffd19d360c9a7faae440b8916/
    http://autobuild.buildroot.org/results/407/40797597024cbb9054fdda8bdf4d011bba966b06/



Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>