doc: do not build

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>

diff -durN openldap-2.4.40.orig/Makefile.in openldap-2.4.40/Makefile.in
--- openldap-2.4.40.orig/Makefile.in	2014-09-19 03:48:49.000000000 +0200
+++ openldap-2.4.40/Makefile.in	2015-01-31 23:12:54.347678913 +0100
@@ -13,7 +13,7 @@
 ## top-level directory of the distribution or, alternatively, at
 ## <http://www.OpenLDAP.org/license.html>.
 
-SUBDIRS= include libraries clients servers tests doc
+SUBDIRS= include libraries clients servers tests
 CLEANDIRS=
 INSTALLDIRS=