net-nds/openldap: fix modules libtool files prune
the prune_libtool_files is run before the modules intall, that will
leave many la files in openldap module directory broken due to main
la file is gone, which will make revdep-rebuild or cave fix-linkage
keep rebuilding net-nds/openldap.
keep module la files removed by default will prevent user from using
.la file in modules config, which is working but should not been
encouraged, such as BUG:600484
Fixes: https://bugs.gentoo.org/537418
(cherry picked from commit
2c2376d9efd3d77424e3c8a626a165daf881e145)
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>