kdc --builtin-hdb should list loadable backends

This fixes the following problems from #210:

 - hdb_ldap doesn't load even when installed correctly
 - loadable hdb backends not listed by kdc --builtin-hdb

Not fixed:

 - hdb_ldap.so not installed in plugin dir

tests/ldap/krb5.conf.in

@@ -3,6 +3,7 @@
 [libdefaults]
 	default_realm = TEST.H5L.SE
 	no-addresses = TRUE
+        plugin_dir = @objdir@/../../lib/hdb @objdir@/../../lib/hdb/.libs
 
 [realms]
 	TEST.H5L.SE = {